Climate change today (Unit 4)

4.1 Greenhouse effects and Impacts of Climate Change

4.1.2 Solar Radiation at Poles and Equator
Because the Earth is a sphere, solar radiation(sunlight) striking it near the equator will be spread-out over a relatively small area, and will have a large heating effect per square metre. At higher latitudes, sunlight will strike the Earth’s surface more obliquely, be spread out over a larger area, and hence have a smaller heating effect per square metre. This is why equatorial regions are hot and polar regions are cold. This temperature difference drives weather, which seeks to minimise this temperature gradient through the general circulation of the atmosphere. One major circulation system is that in which air rises near the equator, moves polewards in the higher atmosphere, sinking in higher latitudes, and blowing equatorwards near the surface as the trade winds. This system is known as the Hadley Cell after its discoverer George Hadley (1685–1768) — after whom the Hadley Centre is also named. During the northern summer, the northern hemisphere is tilted towards the Sun, making the Sun higher overhead (hence solar heating more intense) and the day longer. During the northern winter, the northern hemisphere is tilted away from the Sun, so the Sun appears lower in the sky and the day is shorter. This gives the seasonal cycle of temperature through the year. Climate is the description of the long-term averages of weather, usually taken over a 30-year period. It describes not only the long period averages of temperature, rainfall and other climate quantities indifferent months or seasons, but also the variability from one year to the next.

4.2 What Determines the Temperature of the Earth?
The Earth has global average temperature, averaged across its entire surface and over a long period such as a decade, of about 14°C. Two streams of energy determine what the average surface temperature of the Earth is. Firstly, energy coming in from the Sun as sunlight, which we can see with our eyes. This acts to warm the surface of the Earth and the atmosphere. Secondly, because the surface of the Earth (even on the coldest night in Antarctica) is warmer than outer Space, infrared radiation is everywhere being emitted from the surface of the Earth. This acts to cool the Earth’s surface. We cannot see infrared with our eyes, but it is straightforward to measure with instruments.. The balance between these two streams of energy — that emitted by the Sun and that emitted by the Earth — determines the global average surface temperature of the Earth. If the amount of solar radiation reaching the Earth, or the amount of infrared radiation leaving the ground, changes, then the Earth’s temperature will change. Any change will be very slow because the Earth’s climate system has a large thermal inertia, mainly due to the ocean.

4.3 Greenhouse effects on a Greenhouse
In order to understand the greenhouse effect on Earth, a good place to start is in a greenhouse. A greenhouse is kept warm because energy coming in from the Sun (in the form of visible sunlight) is able to pass easily through the glass of the greenhouse and heat the soil and plants inside. But energy which is emitted from the soil and plants is in the form of invisible infrared (IR) radiation; this is notable to pass as easily through the glass of the greenhouse. Some of the infrared heat energy is trapped inside; the main reason why a greenhouse is warmer than the garden outside. However, this is a rather crude analogy to the way the greenhouse effect works on Earth, as will be seen next. The existence of the greenhouse effect has been known about for a long time. In a paper published in 1896 by the Swedish scientist Arrhenius, he discusses the mean temperature of the ground being influenced by the presence of heat-absorbing gases in the atmosphere. Indeed, the notion of heat absorption by gases was put forward even earlier than Arrhenius’ time, by scientists such as Tyndall and Fourier. So the greenhouse effect may be relatively new to policy makers and the media, but in the scientific community it has been known about, and investigated, for well over 100 years.

4.3.1: Greenhouse effects in the Atmosphere
As explained earlier, the temperature of the Earth is determined by the balance between energy coming in from the Sun in the form of visible radiation, sunlight, and energy constantly being emitted from the surface of the Earth to outer Space in the form of invisible infrared radiation. The energy coming in form the Sun can pass through the clear atmosphere pretty much unchanged and therefore heat the surface of the Earth. But the infrared radiation emanating from the surface of the Earth is partly absorbed by some gases in the atmosphere, and some of it is re-emitted downwards. The effect of this is to warm the surface of the Earth and lower part of the atmosphere. The gases that do this working the natural atmosphere are primarily water vapour (responsible for about two-thirds of the effect) and carbon dioxide. A rather more rigorous explanation is given in the next section. The natural greenhouse effect has operated for billions of years. Without the greenhouse effect due to natural water vapour, carbon dioxide, methane and other greenhouse gases in the atmosphere, the temperature of the Earth would be about 30°C cooler than it is, and it would not be habitable. So the greenhouse effect due to naturally occurring greenhouse gases is good for us. The concern is that emissions from human activities (for example, CO2 from fossil-fuel burning) cause these greenhouse gas concentrations to rise well above their natural levels and as new greenhouse gases (such as CFCs and the CFC replacements) are added, the further warming which will take place could threaten sustainability. This is discussed in later sections

4.3.2 The Greenhouse effect; a more Rigorous Explanation
A more rigorous explanation of the greenhouse effect is as follows. Temperature in the lower atmosphere (troposphere) decreases with height, on average. That for the present day climate is shown by the black line in the above diagram. (Temperature is shown in units of Kelvin (K); 0.°C is about 273 K). The infrared radiation that cools the Earth comes from an average height of about 5.5 km at present, due to absorption and re-emission of greenhouse gases. In a future world with higher greenhouse gas concentrations, emission of IR to space will be from a higher (and therefore cooler) layer. Because the rate of emission of IR increases with temperature, emission from this cooler layer will be reduced, and the atmosphere then warms up (red line) until the rate of IR emission to space reaches the original rate. The surface temperature will then be warmer than that in the present day.

4.3.3 Long-term Natural Climate Changes are likely driven by Earth’s Orbit Changes
It is well known that global temperatures change substantially over timescales of a hundred thousand years, as climate moves from ice ages to interglacials). The actual measurement is of the concentration of deuterium in air bubbles, and this can be related to local temperatures. Swings between glacial and interglacial climate are likely to be initiated by subtle differences in the Earth’s orbit and tilt of axis around the Sun, known as the Milankovic Effect after its proposer. Although these orbital changes dictate that the Earth will enter another ice age, this is unlikely to be for many thousands of years — quite a different timescale to that of man-made warming. Some researchers believe that man-made warming may actually prevent the Earth from entering another ice age.

4.4 Energy from the Sun; Stable over the last 50 years
In addition to long-term changes due to the Earth’s orbit, there are two main natural agents which can change global climate: changes in energy we receive from the Sun and the effect of volcanoes. Solar irradiance before 1978 is estimated from proxy data (sunspots, etc.) and is less reliable than that measured since then by satellites. Based on the Hadley Centre HadCM3 climate model, we can estimate the global temperature increase which the changing solar radiation may have caused; this is shown on the right-hand scale and amounts to one or two tenths of a degree, so may explain at least some of the global temperature rise observed in the early part of the 20th century. (Note that, because of the large inertia of the climate system, global temperature does not respond significantly to the 11-year solar cycle). There are some theories that the solar influence on global climate could be amplified by an indirect route, for example involving stratospheric ozone or cosmic rays or clouds. A review of current understanding was prepared for the Hadley Centre by the University of Reading and Imperial College, London. This concluded that there is some empirical evidence for relationships between solar changes and climate, and several mechanisms, such as cosmic rays influencing cloudiness, have been proposed, which could explain such correlations. These mechanisms are not sufficiently well understood and developed to be included in climate models at present. However, current climate models do include changes in solar output, and attribution analyses that seek to understand the causes of past climate change by comparing model simulations with observed changes, do not find evidence for a large solar influence. Instead, these analyses show that recent global warming has been dominated by greenhouse gas-induced warming, even when such analyses take account of a possible underestimate of the climatic response to solar changes by models.

4.4.1 Change in Volcanic Aerosol
Volcanoes inject gas into the atmosphere. If they are energetic enough, this gas will reach the stratosphere and form small sulphate aerosol particles which can persist for a few years. They reflect back some of the solar radiation which otherwise would have heated the surface of the Earth, and hence act to cool the planet. The amount of volcanic aerosol in the atmosphere is very variable, indicated by this time series of its estimated optical depth, and the cooling effect that this would have. Although energetic volcanoes were relatively common in the late 19th century (for example, Krakatoa in 1883) and early 20th century, and there have been substantial numbers of energetic volcanoes since the 1960s (most recently, Pinatubo in 1991), there was a period in the 1940s and 1950s when the atmosphere was relatively clear of volcanic aerosol. The amount of climate cooling due to volcanic aerosols would have been quite small in that period. This unusually low amount of volcanic cooling (together with the increase in solar radiation) may have contributed to temperatures in the 1940s being relatively high compared to earlier decades. As with solar energy, optical depth due to volcanic aerosols has been estimated indirectly before about 1983, and hence is less certain.

4.5 Emissions of CO2 from Fossil-fuel Burning; Rapid Rise since 1950
In an earlier section, mention was made of emissions from human activities enhancing the natural greenhouse effect. The main gases involved in this are shown in the table below:

Emissions of carbon dioxide into the atmosphere from human activities have increased since the Industrial Revolution, particularly since about 1950. Smaller sources such as cement production and gas flaring are omitted for clarity, but included in the total. In addition to the fossil-fuels source, carbon dioxide is also emitted when forests are cleared and burnt. Figures for this source are less accurate, but the best estimate from IPCC is 1.7 GtC per year during the 1980s.

4.5.1 Natural and Man-made Carbon Cycles
Emissions of CO2 from human activities become involved in the natural carbon cycle, a system of fluxes of CO2 between land (vegetation and soils), ocean (water and ecosystems) and the atmosphere. The IPCC TAR estimated that, averaged over the decade of the 1980s, fossil-fuel burning emitted 5.4 GtC/yr into the atmosphere and land use change (mainly deforestation) a further 1.7 GtC/yr, giving a total of about 7 GtC/yr. The atmosphere retained about 3.3 GtC of this per year, leading to the measured rise in CO2 concentration, with about 1.9 GtC/yr going into each of the two main sinks, the ocean and the land (soils plus vegetation).Many of these estimates are very uncertain. Man-made fluxes are much smaller than the fluxes in the natural carbon cycle. However, the natural carbon cycle is in balance, and has led to concentrations of CO2 in the atmosphere remaining relatively constant for the thousand years before the industrial revolution. Similar cycles (known by the general term of biogeochemical cycles) exist for other greenhouse gases such as methane and nitrous oxide. In the case of methane, emissions from natural and human activities undergo complex chemical interactions in the atmosphere with species such as the hydroxyl radical (OH), concentrations of which are themselves affected by other man-made emissions, such as carbon monoxide. Ozone in the lower atmosphere (troposphere) is also a greenhouse gas. It is formed by atmospheric chemical reactions between man-made emissions such as hydrocarbons, nitrogen oxides, carbon monoxide and methane. In the case of water vapour, which is the most important natural greenhouse gas, emissions from human activity have very little effect on its concentration in the atmosphere, which is mainly determined by temperature (and, hence, human activity does affect it indirectly, via global warming).

4.5.2 Drivers of CO2 emission: Population and Energy Use
The main factors which have caused the rise in CO2 emissions shown in the previous slide are twofold:(a) growth in population and (b) growth in energy use per person, as more people enjoy a more energy-intensive standard of living, with increased ownership of goods, more services and greater travel. Of course, energy use per person is very different from country to country.

4.5.3 Carbon dioxide in the Atmosphere; Rapid Rise due to Human Activities
The concentration of carbon dioxide in the atmosphere was roughly constant at about 280 parts per million (ppm) for 800 years (and probably longer) before
the start of the industrial revolution. We know this from analysis of air trapped in bubbles in ancient ice cores in Antarctica and Greenland. These ice-core samples then show a gradual rise from about 1800, accelerating with time. Direct measurements of CO2 in the atmosphere have been made since the late 1950s, notably by Keeling on Mauna Loa in Hawaii. These measurements have shown a steady rise up to a current annual-average value (2004) of 378 ppm. In addition to the trend, the effects of seasonal cycle of vegetation growth and decay can be clearly seen. Concentrations in the atmosphere of other greenhouse gases have also risen due to human activities. Methane was about 800 parts per billion(ppb) 200 years ago, and is now at more than 1750 parts per billion, although its rise has levelled off, possibly due to reduction in natural gas leakages. Nitrous oxide has risen from a pre-industrial concentration of about 270 ppb to a current level of over 310 ppb. Ozone in the lower atmosphere has a less robust observational long-term record but appears to have increased over the same period by about 30%.

4.5.3 Man-made CO2 has Diluted Natural CO2
How do we know that the rise in carbon dioxide concentrations since the Industrial Revolution is due to emissions from man’s activities? The carbon in CO2 has several different forms; the most common(about 99%) is called 12C, but there is a very small fraction of 14C, which is radioactive, with a half life of about 5,700 years. Because fossil fuels are so old, the 14C in them has decayed, so the CO2 given off when we burn them has very much less 14C in it. So the amount of 14C in the air is being diluted by CO2 emissions from burning coal, oil and gas, known as the ‘Suess effect’. We can estimate the change in 14C in the air from 1850 to 1950 by measuring it in tree rings. When we calculate what this should be, based on man-made CO2 emissions, the calculation (red line) agrees well with the measurements. This is proof that the rise in CO2 concentration is due to fossil-fuel burning. The technique fails to work after about 1950, because radioactivity from atomic bombs corrupts the technique. There is other supporting evidence, such as the consistency between the rise in concentration (unprecedented over the last several hundred thousand years) and man-made emissions, and the north-south gradient of CO2 concentration.

4.5.4 Large Relations in CO2 Emissions are required to Stabilise Concentrations
The long, effective lifetime of carbon dioxide means that its concentration in the atmosphere would be very slow to respond to a reduction in emissions. The red line shows that if future carbon dioxide emissions follows one ‘business as usual’ projection, then its concentration in the atmosphere will roughly double over the next 100 years. If we were able to stabilise global emissions at constant 1990 levels, carbon dioxide concentration in the atmosphere would still go on rising substantially (blue line). And even if emissions were cut in half overnight and continued at that level for 100 years, then carbon dioxide concentrations would still actually creep up (green line). Only by a reduction of about 70% in carbon dioxide emissions would we be able to stabilise its concentration in the atmosphere. That is not the same thing as calling fora cut of 70%; it is simply pointing out that the science of the carbon cycle leads to that conclusion. However, the carbon cycle model used in the calculations above assume no feedback between the climate and the carbon cycle. Slide 60 shows that the two are actually closely coupled, and the resulting feedback may mean that the reduction in emissions required to stabilise carbon dioxide concentrations in the atmosphere would be even larger than 70%.

4.5.5 CO2 is the Major Contributor to Global Warming
We have already discussed CO2, methane, nitrous oxide and ozone as greenhouse gases. Other greenhouses gases such as chlorofluorocarbons(CFC) damage the ozone layer and emissions have, hence, been virtually eliminated as a result of the Montreal Protocol. However, because they have lifetimes of around 100 years, their concentrations in the atmosphere are only now slowly starting to decrease. Equal amounts emitted of each greenhouse gas has a different capacity to cause global warming. This depends upon its lifetime (the longer the emission remains in the atmosphere, the more time it has to exert a warming influence), the amount of extra outgoing infrared radiation it will absorb in the atmosphere, and its density. The future warming effect, usually taken over the next 100 years, of an extra 1 kg of a greenhouse gas emitted today, relative to 1 kg of CO2, is known as its Global Warming Potential (GWP). Current estimates are: methane: 23; nitrous oxide: 296; CFC12: 7300; SF6:22200. CO2 = 1, by definition. The warming effect over the next 100 years of current emissions of the greenhouse gases will depend upon the amount of each gas being emitted globally and its GWP. When this calculation is done, it is seen in this slide that carbon dioxide will be responsible for about two-thirds of the expected future warming. About a quarter of the warming is expected to be due to methane, with other greenhouse gases making up the rest. Carbon dioxide is certainly the most important man-made greenhouse gas, presently and in the future. Note that tropospheric ozone is not included in this calculation.

4.5.6 Sulphur Aerosols Cool Climate Directly and Indirectly
Very small particles, known as aerosol, have a substantial effect on climate. Sulphate aerosol in the lowest part of the atmosphere (the boundary layer) is created when sulphur dioxide, emitted by human activities such as power generation and transport, is oxidised. Sulphate aerosol particles scatter some sunlight, which would otherwise reach the surface of the Earth and heat it, back out to Space. They therefore have a cooling influence on climate. The amount of sulphate aerosol in the atmosphere has increased by three or four times over the past 100 years or more. Sulphate aerosol particles also have a further, indirect, effect on climate, Clouds are generated when air becomes saturated with water vapour and water condenses onto small particles (cloud condensation nuclei) to form cloud droplets which reflect some sunlight. In a polluted lower atmosphere, because there are more cloud condensation nuclei, then for the same amount of water we get clouds which have a larger number of smaller droplets. These have a greater surface area and, hence, will reflect back more sunlight than clouds in clean air. This gives an additional, indirect, cooling effect of aerosols. The story is further complicated by the ability of aerosols to change the lifetime of the cloud. Smaller droplets are less likely to coalesce to form drizzle drops. So clouds in a polluted atmosphere will persist longer than their clean-atmosphere equivalents, and the consequent greater cloud cover will also exert a cooling influence. Other types of aerosol particles can have different effects on climate. Black carbon (soot) emitted from (incomplete) burning of fossil fuels, such as in diesel smoke, will absorb solar radiation and cause the atmosphere to heat up. IPCC estimates that this may have caused about 7% of the man-made heating effect since pre-industrial times. Windblown dust, inorganic carbon and sea-salt aerosols are also important. Aerosols in the boundary layer have a lifetime of only a few days, since they caneasily be washed out by rain or incorporated into clouds which subsequently rain.

4.5.7 Estimated Burden of Sulphate Aerosol
The amount of sulphate (SO4) aerosol in the atmosphere, averaged over the decade of the 1990s and calculated by the latest Hadley Centre climate model (HadGEM1), is shown in this slide. Sources considered are sulphur dioxide from human activities, ‘background’ non-explosive volcanoes, and natural di-methyl sulphate (DMS) from ocean plankton. The model takes these, considers the effect of chemical processes in the atmosphere and physical processes such as dry and wet deposition, and deduces the resulting concentration of sulphate aerosol, expressed here as a column burden in milligrams per square metre. Because aerosols only remain in the atmosphere fora few days, the highest atmospheric concentrations are estimated to be downwind of the greatest emissions areas in Asia. DMS is an important contributor only to oceanic areas away from human activities. This uneven geographic distribution means that sulphate aerosols have a complex effect on climate, both locally and globally. Sulphur dioxide gas and sulphate aerosols in the atmosphere can have impacts on human health, and also lead to acid rain which can fall at considerable distances from the emissions, acidifying waters such as lakes and endangering ecosystems. For these reasons, sulphur emissions have been greatly reduced in the US and Europe since the 1980s. It is expected that similar considerations will, in due course, lead to reductions of sulphur emissions in Asia and other rapidly developing parts of the world. When this happens, the current cooling effect of sulphate aerosols will be reduced, producing a warming effect which would add to that from greenhouse gases.

4.6 Man-made Greenhouse Gases Dominate the Change in Climate Forcing
The external agents which act to change the climate of the Earth, such as greenhouse gases and solar radiation discussed in earlier slides, are known as forcing agents. The change in the energy available to the global Earth- atmosphere system due to changes in these forcing agents is termed the radiative forcing of the climate system and has units of watts per square metre (Wm-2). Thus, the radiative forcing is an index of the relative global mean effect of various agents on the climate of the Earth’s surface and lower atmosphere. This slide, taken from the IPCC TAR, shows the change in radiative forcing over the period 1750 to 2000, due to a number of forcing agents, each of which is linked to human activity (except for solar radiation). In some cases (for example, tropospheric ozone) a best estimate is shown, together with vertical error bar showing the range of estimates. In other cases, such as mineral dust, only a range of uncertainty can be given. The level of scientific understanding of each of the factors is shown along the bottom of the diagram. Man-made changes in greenhouse gas concentrations represent the biggest and best-understood effect on climate over the period, as shown on the far left of the diagram, and carbon dioxide is the biggest contributor to this.

4.6.1 Stages in Predicting Climate Change
How quickly the climate will change in the future depends upon two factors: how much greenhouse gas emissions grow, and how sensitive the climate system is to these emissions. We predict future climate change in a number of stages, shown in this figure. The first thing we need to estimate is the future emissions of greenhouse gases and other gases which affect climate change. These projections are deduced from separate models which take into account population growth, energy use, economics, technological developments, and so forth. We do not carry out this stage at the Hadley Centre, but we take future scenarios of these emissions from others, particularly the IPCC in its Special Report on Emissions Scenarios (SRES). Having obtained projections of how emissions will change, we then calculate how much remains in the atmosphere, i.e. what future concentrations will be. For CO2, this is done using a model of the carbon cycle, which simulates the transfer of carbon between sources (emissions) and sinks in the atmosphere, ocean and land (vegetation). For gases such as methane, we use models which simulate chemical reactions in the atmosphere. Next we have to calculate the heating effect of the increased concentrations of greenhouse gases and aerosol particles, known as radiative forcing. This is relatively straightforward because we know their behaviour quite well from laboratory studies. Finally, the effect of the changed heating on climate has to be calculated. This complete pathway, from emissions to concentrations to heating effect to climate change, can be done within the climate model, described shortly, which can predict changes in spatial patterns of climate quantities such as temperature at the Earth’s surface and through the depth of the atmosphere and oceans. The additional heating of the climate system which would occur if the concentration of CO2 in the atmosphere was doubled, is about 3.8 Wm-2. In a simple world this would ultimately warm the surface by about 1°C. The prediction of climate change is complicated by the fact that, once climate change starts, there will be consequences (feedbacks)in the climate system which can act to either enhance or reduce the warming. For example, as the atmosphere warms it will be able to ‘hold’ more water vapour. Water vapour itself is a very powerful greenhouse gas, so this will act as a positive feedback and roughly double the amount of warming. Similarly, when sea ice begins to melt, some of the solar radiation which would otherwise be reflected from the sea ice is absorbed by the ocean, and heats it further; another positive feedback. On the other hand, when carbon dioxide concentrations increase in the atmosphere then it acts to speed up the growth of plants and trees (the fertilisation effect)which in turn absorb more of the carbon dioxide; this acts as a negative feedback. There are many of these feedbacks, both positive and negative, many of which we do not fully understand. This lack of understanding is the main cause of the uncertainty in climate predictions; this applies in particular to changes in clouds which we will return to later. Following on from the climate change prediction, the impacts of climate change, on socio- economic sectors such as water resources, food supply and flooding, can be calculated. These is usually done by supplying climate change predictions to other, off-line, impacts models, and Hadley Centre data have been used by hundreds of impacts researchers in this way. At the Hadley Centre we are also incorporating some impacts models into the climate model itself, as this has many advantages.

4.6.2 The Climate System
In order to estimate climate change, we have to build a mathematical model of the complete climate system. Firstly, the atmosphere; the way it circulates, the processes that go on in it, such as the formation of clouds and the passage of terrestrial and solar radiation through it. Secondly, the ocean, because there is a constant exchange of heat, momentum and water vapour between the ocean and atmosphere and because in the ocean there are very large currents which
act to transport heat and salt. In fact, the ocean does about half the work of the climate system in transporting heat between the equator and the poles. Thirdly, the land, because it affects the flow of air over it, and is important in the hydrological (water) cycle. In addition, we model the cryosphere; ice on land and sea. All of these components of the climate system interact to produce the feedbacks which determine how climate will change in the future.

4.6.3 Climate Models Continue to be Improved
The climate model is a mathematical description of the Earth’s climate system, broken into a number of grid boxes and levels in the atmosphere, ocean and land, as shown above. At each of these grid points in the atmosphere (for example) equations are solved which describe the large-scale balances of momentum, heat and moisture. Similar equations are solved for the ocean. The atmospheric part of the third Hadley Centre coupled ocean-atmosphere climate model, HadCM3 — many results from which are shown in this presentation — has a grid of 2.5° latitude x 3.75° longitude, and has 19 vertical levels. The ocean model has 20 vertical levels and a grid size of 1.25° latitude x 1.25° longitude. In all, there are about a million grid points in the model. At each of these grid points equations are solved every half hour of model time throughout a model experiment which may last 250 or, in some cases, 1,000 years. The Met Office currently uses a NEC SX-6/SX-8super computer. The HadCM3 model is run typically for 250 years of simulation (1850–2100) taking about three months’ clock time on one SX-6 node. Several simulations can be run at the same time. Currently, some 300 terabytes (million million bytes) of data are stored for future analysis. This is expected to double each year. The Hadley Centre has just completed development of its new climate model, known as the Hadley Global Environment Model (HadGEM1). As can be seen above, this has a higher resolution horizontally and vertically, on land and over the oceans. It also incorporates improvements to the representation of the dynamics of the atmosphere, and of many processes in the atmosphere and oceans. Because of this, it is about 15 times more expensive to run than HadCM3.

4.6.4 Global Warming Trends not due to Urbanisation
Despite the careful corrections that are made to temperature observations to account for urbanisation, concerns still remain that part of the rise in land temperatures seen over past decades is due to changes in or around observing sites; for example, where a town is creeping out towards a previously rural area where the climate observation is made. We know that the climate of an urban station differs greatly from the surrounding countryside; the so-called urban heat island (UHI) effect. In the night, temperatures do not fall as quickly in cites, as the mass of concrete helps to retain heat, and city surfaces cannot radiate heat away as fast as in the country. The UHI effect is weakened or destroyed when there is a strong wind. So if the recently observed warming is an artefact of urbanisation, then one might expect the temperature rise on calm nights to show this increasing urbanisation and, hence, to be more rapid than that on windy nights. Recent Hadley Centre work has used daily night-time minimum temperature measurements at more than 250 land stations over most of the world, during the period1950–2000. Data were taken for the top third most windy nights and the bottom third least windy nights (that is, the most calm conditions). Trends for these two subsets are plotted in the slide. Although, as expected, minimum temperatures in windy conditions are somewhat higher, the trend is the same in windy and calmer conditions. This clearly demonstrates that warming over the past 50 years has not been due to urbanisation.

4.6.5 Natural Factors cannot Explain Recent Warming
What are the causes of changes in global mean temperature observed since the early 1900s. As we have already outlined, natural factors include a chaotic variability of climate due largely to interactions between atmosphere and ocean; changes in the output of the Sun and changes in the optical depth of the atmosphere from volcanic emissions. The Hadley Centre climate model has been driven by changes in all these natural factors, and it simulates changes in global temperature shown by the green band in the slide above. This clearly does not agree with observations, particularly in the period since about 1970 when observed temperatures have risen by about 0.5°C, but those simulated from natural factors have hardly.

4.6.6 Recent Warming can be Simulated when Man-made Factors are Included
If the climate model is now driven by changes inhuman-made factors — changes in greenhouse gas concentrations and sulphate particles — in addition to natural factors, observations (red) and model simulation (green) are in much better agreement. In particular, the warming since about 1970 is clearly simulated. Of course, this agreement may, to some extent, be fortuitous, for example, if the heating effect of man-made greenhouse gases and the cooling effect of man- made aerosols have been overestimated. Nevertheless, the ability to simulate recent warming only when human activities are taken into account is a powerful argument for the influence of man on climate. Since this initial Hadley Centre experiment, other modelling centres have been able to reproduce the same broad conclusion. In addition to simulating the global mean temperature, the model also simulates the pattern of changes in temperature, across the surface of the Earth and through the depth of the atmosphere. These ‘fingerprints of man-made warming’ have been compared to observations, providing even stronger evidence for the majority of the long-term trend over the last 50 years having been due to human activity.

4.6.7 Warming in the Atmosphere
Temperatures are routinely measured not only at the Earth’s surface and in the oceans, but in the atmosphere too. The two panels above show changes in temperatures in the lower stratosphere(roughly 12 km–18 km) and the lower troposphere(from the surface to roughly 7 km), relative to the period 1981– 1990. Atmospheric temperatures are measured using two very different techniques: firstly using thermometers carried aloft on routine weather balloons (radiosondes) and, secondly, since 1978, by remote sensing from a Microwave Sounding Unit (MSU) carried on a series of satellites. Measurements from both techniques are subject to corrections and different methods of analysis. The radiosonde data above (black) is from the Hadley Centre analysis known as HadAT2. The stratosphere (top panel) has cooled on average by about 1.5°C since the late 1950s. This arises from several causes. Firstly, does not warm the stratosphere as it does the troposphere; instead it radiates away heat to outer Space and cools it. Thus, increases in CO2 will have led to a greater cooling influence. Secondly, ozone in the stratosphere is heated by solar radiation and raises the temperature there; because stratospheric ozone has decreased l(due to man-made CFCs) this will also act to cool the stratosphere. Lastly, some cooling is suspected to be due to increased concentrations of water vapouring the lower stratosphere, which radiates away heating the same way as CO2. The long-term cooling trend has been punctuated by spikes where aerosol from volcanoes Agung, El Chichon and Pinatubo absorbed solar radiation and thus produced strong stratospheric warming for two or three years. The lower panel
shows the change in global-average lower tropospheric temperature measured by radiosondes and satellite. Also shown in green is the change in global surface temperature. It can be seen that, since 1978, the longer term trend and the variability of two of the three measurements of tropospheric temperature agree well, and also agree with the surface observations. The alternative(UAH) analysis of satellite data shows substantially less warming than at the surface. Climate models predict that we should have seen a relatively greater warming in the troposphere than at the surface; this potential discrepancy between model sand observations is not well understood, although uncertainty in observations is the more likely explanation. The topic of stratospheric and tropospheric temperature measurements is currently undergoing a thorough review in the US, with the involvement of Hadley Centre staff. The conclusions are due out in early 2006.

4.7 Tropical Storm
There appears to be no clear trend in the global frequency of tropical cyclones, also known as hurricanes in the North Atlantic and typhoons in the Pacific. However, research at MIT has shown that the total power dissipated by tropical storms in both these regions, integrated over their lifetimes, has increased markedly since the mid-1970s. This trend is due to both longer storm lifetimes and greater storm intensities, and is highly correlated with change in tropical sea-surface temperatures(SSTs). The figure above shows smoothed changes in the power dissipated by hurricanes (Power Dissipation Index, PDI) in the western North Pacific and North Atlantic areas, integrated over the lifetime of the storm and over its area, compared to changes in the annual mean sea-surface temperature in the Hadley Centre Sea-Ice and Sea Surface Temperature (HadISST) data set, averaged between 30°S and 30°N. The units of PDI are those of energy, with units multiplied by an arbitrary factor to match the same units as the change in SST, to facilitate the comparison between the two quantities. The power dissipation appears to have nearly doubled over the past 30 years. Based on theoretical considerations, only part of the observed increase in power dissipation is likely to be directly due to changes in SSTs. Other factors which are known to influence development are wind shear and the depth of the warm water layer. A recent survey has shown that over the last 35 years there had been roughly a doubling of the number of hurricanes in the two most intense categories (known as Saffir-Simpson 4 and 5, having wind speeds over 56 m/s), with the largest increase sin the North Pacific, Indian and Southwest Pacific Oceans, and the smallest in the North Atlantic. In both these papers, relatively short periods of observational records are used. The role of natural cycles of hurricane activity, and the balance between natural and man-made influences in the recent record, are still a matter of debate. In order to attribute recent observed changes to human activity, in the context of substantial natural variability, further research, ideally involving a longer observational record would be required. There is mounting theoretical and modelling evidence that tropical cyclones will become more intense, although not necessarily more numerous, in a warmer world.

4.8 Components of Sea-level Rise
There is substantial interest in the effects of climate change on sea level, as the increased risk of coastal flooding could markedly affect society. Sea level will change due to expansion of oceans as they warm, and due to the influx of water from melting of glaciers and other snow and ice, and changes in the two large ice sheets in Antarctica and Greenland. Above is a plot of changes in sea level predicted by the Hadley Centre model from the years 1860 to 2100, due to each of these contributors, under a Medium-High Emissions scenario.. Snowfall is predicted to increase over Antarctica in the Hadley Centre model, and this will act to reduce sea-level rise. Over the next century or so, the effect of Antarctic melting may overcome any reduction in sea level due to increased snowfall and make Antarctica a net contributor to sea-level rise. Other models show different estimates for the components and for the total, indicating the need to improve descriptions of all the included processes, and the inclusion of additional ones such as permafrost melt and man-made storage in reservoirs.

4.8.1 Long-term Commitment to Sea-level Rise
Greenhouse effect heating in the atmosphere is rapidly transferred into surface ocean waters. It then slowly penetrates deeper and causes more and more of the ocean depth to expand and, hence, leads to further sea-level rise. This figure shows the sea-level rise due to ocean thermal expansion, estimated from a
climate model experiment where CO2 concentration in the atmosphere was hypothetically increased by 1% per year from time zero to 70 years (that is, until it had doubled) and was then stabilised at that concentration, that is, no further increase occurred. The initial blue line shows thermal expansion while the CO2 concentration was rising, the continuing red line shows sea-level rise after CO2 concentration had been stabilised. Despite the fact that CO2 in the atmosphere did not change after year 70, the sea level carries on rising for many hundreds of years, with only a slow decrease in the rate of rise. So at any time the sea- level rise caused by the man-made greenhouse effect carries with it a commitment to an additional, inescapable, rise.

4.8.2 Oceans are Predicted to become more Acidic
The oceans of the world naturally exchange large amounts of carbon dioxide with the atmosphere. Where oceans are warm, they outgas CO2 into the atmosphere; where they are cold they take up CO2 from the atmosphere. The global ocean is also a major sink for man-made carbon dioxide, currently responsible for absorbing about a quarter of it. The increase in concentration of CO2 in surface waters produced by the absorption of man-made CO2 will reduce the chemical uptake of CO2 from the atmosphere; a reduction in this sink will tend to leave more CO2 in the atmosphere and act to speedup warming (although changes in biological processes, which also control the strength of the ocean sink for CO2, may offset these chemical changes). Increase in ocean surface temperatures due to human activity will also result in less uptake of CO2 from the atmosphere. Basic chemistry tells us that when atmospheric CO2 is absorbed it acidifies the ocean water. The Hadley Centre coupled climate-carbon cycle model has been used to predict how the surface pH of the ocean will change over the period 1860–2100, and the result is shown in this slide.. There is convincing evidence to suggest that acidification will affect calcification, the process by which animals such as corals make shells from calcium carbonate, and this will threaten tropical, subtropical and even cold-water corals. Phytoplankton and zooplankton, which are a major food source for fish, may also be affected. Recent work concludes that key marine organisms — such as corals and some plankton — will have difficulty maintaining their external calcium carbonate skeletons. Indications are that conditions detrimental to
high-latitude ecosystems could develop within decades, not centuries as suggested previously.

4.9 West Antarctic and Greenland Ice Sheet
There are two major ice sheets which are thought to be vulnerable to climate change: the West Antarctic Ice Sheet (WAIS) which contains ice equivalent to about 6m of global sea-level rise, and the Greenland Ice Sheet containing just over 7m of sea-level equivalent. Very much larger amounts of water are locked up in the remainder of the Antarctic ice sheet, but this is not thought to be vulnerable to a warming of less than about 20°C. In the case of the Greenland Ice Sheet, if summer regional temperatures were to rise by about 3°C, the ice sheet would begin to reduce in size. It would be slow to disappear, perhaps half of it taking about 1,000 years to melt. This critical temperature is predicted to be reached by the end of the century by most combinations of climate models and future emissions scenarios. The West Antarctic Ice Sheet is grounded below sea level. Its potential to collapse in response to future climate change is still the subject of debate and controversy. The IPCC TAR took the view that a loss of grounded ice leading to substantial sea-level rise from this source was very unlikely during the 21st century. However, recent measurements suggest that the melting of some ice shelves (floating sea ice attached to the coastline) is leading to a speedup of glaciers and, hence, an increase in their discharge into the sea. The implications of this, needed before reliable predictions can be made, have yet to be understood.

4.9.1 Ocean circulation in the North Atlantic
There have been some concerns expressed that global warming could lead to massive changes in ocean currents such as the Gulf Stream. Currents in the ocean are responsible for about half the work of the climate system in redistributing heat between the equator and the poles. The current system in the N Atlantic is driven by ‘convection’ which takes place in two areas, near Labrador and in the Greenland-Iceland-Norway sea. Here, the surface water is cooled by arctic winds, and sinks a few thousand metres to the bottom of the ocean. This cool dense water then flows southwards, with a flow equivalent to a hundred Amazon rivers, crossing the equator and heading south. The sinking cold water in the north has the effect of drawing northwards warm near-surface water from the Gulf of Mexico, which travels across the North Atlantic. It is often called the Gulf Stream, but is more properly referred to as the North Atlantic Drift. The heat which it transports towards north-west Europe is part of the reason why countries such as the British Isles and Norway are a lot warmer than, for example, those parts of western Canada at the same latitudes. This global ocean circulation, which extends to other oceans of the world, is known as the thermohaline circulation (THC), as it is driven by differences in temperature and salinity of the water masses.

4.10 Decline of Global Soil and Vegetation Sinks
In an earlier slide we saw that, averaged over the 1980s, of the 7 GtC/yr emitted from human activities, about 3 GtC/yr remained in the atmosphere and about 2 GtC/yr was absorbed by the oceans and by vegetation on land. The two ‘sinks’ together take up about half of our emissions and moderate CO2 concentrations and reduce man-made global warming. However, this may not always be the casein future. As global temperatures rise, and rainfall patterns change, we believe that several changes to carbon absorption will take place. Firstly, in the right conditions, CO2 fertilises vegetation and speeds up its growth; this will absorb more of our CO2. Secondly, higher temperatures and more rainfall will encourage growth of high latitude forests, and this will also help to mop up more of our CO2 However, as soils get warmer, the microbial action which breaks down humus works faster, and this will cause more CO2 to be emitted into the atmosphere. Lastly, higher temperatures (and thus higher evaporation) and lower rainfall are predicted for some forests in the tropics, and this is predicted to cause them to die back, with their carbon store being returned to the atmosphere. The Hadley Centre coupled climate-carbon cycle model has been used* to estimate changes to the amount of carbon stored in oceans, vegetation and soils. The reduction in natural carbon sinks, and their eventual change into carbon sources, allows more man-made CO2 to remain in the atmosphere, and so concentrations build-up faster — a positive feedback between the climate and the carbon cycle. Under one business-as-usual emissions scenario CO2 concentration in the Hadley Centre model was predicted to rise to 750 ppm by 2100, accompanied by a warming over land of about 5°C. When the feedback between climate and the carbon cycle was included in the model, CO2 was predicted to rise to 1,000 ppm, and global mean temperature overland to 8°C, under the same emissions scenario. This first estimate of the strength of the feedback has since been repeated by other modelling centres, which unanimously agree that climate change will reduce the natural absorption of CO2 by the biosphere, although they find a variety of different strengths, so more research is needed to reduce uncertainties. But the potential for enhancement of global warming from this feedback has been clearly demonstrated.

 

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