Global Warming: The Climate Science Perspective
Barrie Pittock Honorary Fellow, CSIRO Marine and Atmospheric Research Summary The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body that appoints scientists to summarise the state of the literature on climate change. While it has rigorous procedures to ensure reliability, it necessarily reflects a fairly conservative view of the science. The key problem is the IPCC tendency to focus on the “best estimates” rather than risk management, which requires more attention to the worst possibilities that need to be avoided through policy action. This paper summarises the IPCC conclusions in early 2007 but goes on to point to various understatements of the risks. Evidence for this comes from observations pointing to climate change and sea-level rise occurring more rapidly than the climate models have suggested and mechanisms not yet in the climatic and glaciological models. The uncertainties are large, with both positive and negative feedbacks (amplifying or stabilising respectively) being poorly quantified. Potential impacts are serious, especially regarding water supply, sea level rise, fire frequency and intensity, health and various extreme weather phenomena. Policy relevant advice must be to invest in adaptation to try to cope with changes that are unavoidable, and to reduce greenhouse gas emissions rapidly to minimise future climate change impacts. IPCC Role & Problems IPCC provides the most authoritative review of the state of climate change science, impacts and policy options available. It is an intergovernmental body set up by the World Meteorological Organisation and the United Nations Environment Program with the task of providing policy-relevant but not policy-prescriptive advice. Member governments appoint qualified experts to summarise the refereed and so-called “grey” literature regarding climate change. The expert-authors must agree to the final report, but government representatives also have to approve the final Summary for Policymakers (SPMs) line-by-line in plenary session with the authors of each of the three Working Groups. This often leads to changes in emphasis and omissions from the final SPMs if these are sensitive for policymakers. Problems arise from this process due to several factors: • Vested interests harboured by countries heavily reliant on fossil fuels for industry and development, or for export, lead to pressure to remove worst case estimates, especially in the SPMs. • Scientists, particularly those involved in Working Group I on the physical science, tend to focus on “best estimates”, which they consider most likely, rather than worst cases that may be serious but which have only a small probability of occurrence. Risk
• •
is the combination of probability and impacts, so large potential impacts with low probability should not be ignored. Many scientists prefer to focus on numerical results from models, and are uncomfortable with estimates based on known but presently unquantified mechanisms. Due to the long (four-year) process of several rounds of drafting and peer and government review, an early cut-off date is set for cited publications. In the case of the 2007 reports, this is generally about the middle of 2006. Many important new studies and papers have appeared since then.
The central problem here is that the most policy-relevant advice must be couched in a risk management framework. Uncertainties are admittedly large. This allows some socalled sceptics (really contrarians) to focus on the low end of the uncertainty range and claim there is nothing to worry about, pure scientists to focus on the “best estimates” near the middle of the range of uncertainty, and the applied scientists, engineers and managers to focus on the high impact risks that have to be avoided. Policy-relevant advice alerts decision makers to what must be avoided by good policy. This is of course exactly what we do in insuring a house against fire: it is not that we are sure our house will burn in the next year, but that it just might. Bridges, drains and dams are designed to cope with the worst plausible storm or river flow (a 1 in 100 or 1 in a 1000 chance). This is what climate change policy must do. Key conclusions from the IPCC in 2007 There is no clear and prominent statement in the Working Group 1 (“The Physical Science Basis”) SPM (IPCC, 2007a), but hidden in Table SPM-3 is the full range of projected warmings by 2100. This is 1.1 to 6.4ºC relative to 1990, for the full range of emissions scenarios that IPCC adopted before its previous report in 2001. Figure 1 (Figure SPM-5), shows graphs for the global warming out to 2100 for various emissions scenarios. At the right are vertical grey bars representing the range of uncertainty by 2100 for each of the so-called “marker scenarios”, including the A1FI, or fossil fuel intensive, scenario. These bars have central estimates in colour, and the grey range is that of uncertainty, depending on which climate model and which assumptions are used. Note the curious fact that the graph for the fossil fuel intensive scenario is missing, even though this is the emissions scenario which actual emissions are at present following, and which is therefore highly policy-relevant. Lack of this curve makes interpolation to the full range of possible impacts before 2100 difficult. IPCC (2007a) estimates of possible sea-level rise out to 2100 are also included in the Working Group I SPM, Table SPM-3. The full range is not stated as such, but from the table it is 18 to 59 cm at 2090-2099 relative to 1980-1999. This is in contrast to the full range estimated in the IPCC report in 2001, which was 9 to 88 cm. by 2100. Direct comparison is complicated as exact time intervals are slightly different, but broadly still cover the 21st century. But more critically, the IPCC (2001) report included an estimate for effects of ice sheet dynamics, which is not included in the new table. Instead the omission of an estimate for effects of ice sheet dynamics has been relegated to a couple
of dot points. The first states that “Models used to date do not include uncertainties in climate-carbon cycle feedback nor do they include the full effects of changes in ice sheet flow, because a basis in published literature is lacking.”
Figure 1. The IPCC Working Group 1 Figure for global average surface warming estimates out to 2100. This is Figure SPM-5 in the IPCC report (IPCC, 2007a). The second dot point does discuss a possible contribution form “increased ice flow from Greenland and Antarctica at the rates observed for 1993-2003, but these flow rates could increase or decrease in the future.” It goes on to say that if the rate of outflow were to increase linearly with global warming there could be an extra 10 to 20 cm of sea level rise by 2100, and that “larger values cannot be excluded”. This is despite an extensive literature on the mechanisms that lead to increases in ice flow in outlet glaciers and evidence that a non-linear acceleration in outflow glaciers is happening in both Greenland and Antarctica (see my review published in EOS in August 2006, Pittock, 2006, and the more detailed review in Pittock, 2007). Hansen (2005, 2007) argues that this could mean sea-level rise by 2100 could be as much as 5 m. The latter is not a consensus view, but it is possible. In addition, Rahmstorf ( 2007) has published an estimate based on a linear extrapolation from observed sea-level rise last century, which exceeds the highest IPCC estimate. The various sea-level rise estimates are summarised in Table 1 below.
Table 1. Projections of sea-level rise from the IPCC reports in 2001 and 2007, and other estimates by Rahmstorf (2007) and Hansen (2005, 2007), relative to 1990. Warming to 2100 (ºC) IPCC (2001) full range 1.4 to 5.8 Case Sea-Level Rise to 2100 (cm) 9 to 88 (allows for ice sheet dynamics) 18 to 38 (dynamics caveat only)
IPCC (2007a) B1 (low 1.1 to 2.9 case) IPCC (2007a) A1FI (high case) Rahmstorf (2007) 2.4 to 6.4
Hansen (2005, 2007)
26 to 59 (dynamics caveat only), plus possible 10 to 20 due to a linear increase in ice outflow from Greenland & Antarctica 1.4 to 5.8 (assumed 50 to 140 (assumes linear relation on basis of IPCC, between warming & sea-level rise) 2001) Several meters (assumes exponential relation between warming & sealevel rise)
IPCC (2007a) also published graphs of observations of global mean surface temperature changes, increases in global average sea level and observations of snow cover in the Northern Hemisphere for March-April each year. This IPCC Figure SPM-3 is reproduced here as Figure 2. A selection of key statements from the IPCC (2007a) report follows: Greenhouse gas concentrations now far exceed pre-industrial values, due in the case of carbon dioxide primarily to fossil fuel use and land use change, and in the case of methane and nitrous oxide to agriculture. Global warming is now unequivocal as evident from many different lines of evidence. There is now very high confidence that the net effect of human activities since 1750 has been one of warming, although particulate pollution has led to some smaller regional cooling. The latter is decreasing as particulate pollution is brought under control. The warming contribution from solar variations is about 20 times smaller than that from greenhouse gases. (The solar contribution has in fact reversed in recent decades – see Lockwood and Frohlich, 2007.)
Climate sensitivity, defined as the eventual stablised global warming due to a doubling of atmospheric carbon dioxide concentrations, is likely to be in the range 2 to 4.5ºC, but could be substantially higher. The oceans are likely to become more acidic with the pH value down by between 0.14 and 0.35 by 2100. Global warming and sea-level rise will continue for centuries after greenhouse gas concentrations are stabilized.
Figure 2. Trends in global average surface temperature, sea level and northern hemisphere snow cover since the start of good measurements. This is Figure SPM-3 in the IPCC (2007a) report.
Observations show climate change and sea-level rise are happening faster than expected Actual measurements show that global carbon dioxide concentrations, global average surface temperatures and global average sea-level rise are increasing near the top end of the range of uncertainty set out in the IPCC (2001) report. This is shown in Figure 3.
Figure 3. Changes in key global climate parameters since 1973, relative to 1990 values, compared with the scenarios of the IPCC (2001) report (shown as dashed lines and grey ranges). Top to bottom: observed carbon dioxide concentrations, observed global surface temperatures (from two independent assessments), and observed sea-level rise from tide gauges (red) and satellites (blue). Figure from Rahmstorf et al., 2007, with permission from Science magazine.
These observations suggest that rather than actual changes being near the middle of the accepted IPCC (2001 and 2007a) ranges of uncertainty, they are tracking near the top of the range, meaning that societal changes determining emissions, the climate sensitivity, and perhaps other mechanisms may be higher or worse than thought. A number of processes that have been observed tend to explain the rapid changes (Pittock, 2006, 2007): 1. The climate sensitivity is medium to high (deduced from paleoclimatic and other data). 2. Cooling by particulates (“global dimming”) is decreasing. 3. Permafrost or frozen ground at high latitudes is melting (an amplifying effect as melted permafrost regions will absorb more solar energy). 4. Plants and plant matter in the soil (biomass) are tending to become a source of carbon dioxide or methane rather than a sink. This is due to increasing heat stress, fire and decomposition of humus, and is a weak amplifying effect. 5. Arctic sea ice is retreating, which is another amplifying effect as it allows more solar energy to be absorbed. 6. Changes in air and sea circulations are causing more droughts in mid-latitudes and transporting more heat polewards. 7. Antarctic ice shelves are breaking up, allowing greater outflow from glaciers, and the outlet glaciers are accelerating. 8. Greenland outlet glaciers are accelerating due to penetration of surface meltwater and melting from below where the glaciers are floating in valleys, the floors of which are below sea level. Graphic examples of these processes are available from satellite photos and other observations not reproduced in this paper. Impacts on Australia Australia is one of the most vulnerable countries in the developed world due to its relatively low latitude and aridity. Low latitude and relatively low elevations mean that there is little room to move when temperatures become higher. Moreover, global warming generally moves the belt of westerly winds in mid-latitudes to higher latitudes, so that the southern parts of the country, which rely on winter rains from low pressure systems and cold fronts experience less rainfall as well as greater evaporative losses from higher temperatures. Impacts will include: Greater aridity in the south and east, affecting agriculture, town and city water supplies, more frequent and severe “droughts”, increased fire frequency and intensity, and resulting increases in soil erosion and ecosystem losses
Huge impacts on rapidly developing coasts, via damage to infrastructure, saline intrusion and loss of coastal wetlands from sea-level rise, storm surges and coastal erosion. Changing coastal currents and wave patterns may also change alongshore sediment drift, which will realign many coasts between headlands. It is already true, but little understood, that sea level is rising on average several mm per year now, and this is expected to accelerate to one cm or more per year later this century. Increased heat stress affecting crops, forests and people. Many species and ecosystems are threatened, especially coral reefs, coastal wetlands and mountain plant and animal communities. Floods and storms will become more severe causing damages to infrastructure, erosion and loss of livelihood. Acidification of the oceans will affect reefs and fisheries. Impacts from overseas, including on terms of trade, low-carbon rules, displaced people and possibly instability and conflict in neighboring countries (Dupont and Pearman, 2006). Policy outcomes Clearly, adaptation will be necessary to cope with changes that are already in train and cannot be avoided. There may be large delays due to long time lags in global warming due to the large heat capacity of the oceans, potential instabilities in the major ice sheets, and slow societal change to reduce greenhouse gas emissions. Some changes set in train now or in the near future may be irreversible. However, it is clearly urgent to reduce emissions to minimize climate change and sealevel rise. The IPCC (2007b) report suggests that impacts of global average warmings in the range of 2 to 3ºC are likely to lead to large impacts that many might consider critical or, in subjective terms “dangerous”. IPCC (2007c) finds that stabilising greenhouse gas concentrations at 450 parts per million CO2 equivalent (that is treating other greenhouse gases as if they were the equivalent amount of CO2) would lead to global average warmings in the range of about 1.4 to 3.2ºC, with more than a 50% chance of exceeding 2ºC (see Figure 4). To achieve even such a target greenhouse gas concentration would require greenhouse gas emissions to be eventually reduced by 80% or more. Indeed the IPCC in the 2007 Working Group 3 summary (IPCC, 2007c) states that this may require transient reductions of emissions to negative amounts, i.e., that CO2 may need to be taken out of the atmosphere, e.g., by using biomass to produce energy, with the resulting CO2 being captured and sequestered underground. The present Kyoto Protocol targets are far less ambitious and finish in 2012. However, under the Kyoto Protocol, mechanisms have been pioneered that may form the basis for stricter measures after 2012 that may approach what is needed.
Similarly, the Asia-Pacific Partnership (AP6) if fully implemented, would do little to solve the problem as such. According to the Australian Bureau of Agricultural and Resource Economics (ABARE, 2006), at p.64, AP6 would lead to a 100% likelihood of 1.7°C warming by 2100 relative to 1990, and a 50% chance of more than 2.6°C. Clearly more drastic action is necessary.
Figure 4. The relation between stabilization levels for greenhouse gas concentrations and global warming. The three curves are for climate sensitivities of 1.5, 3 and 4.5ºC (bottom to top), and the Roman numbers refer to groups of stabilization scenarios. From IPCC (2007c), figure SPM-8. A recent paper by Raupach and others in 2007 highlights the current problem, with data indicating that CO2 emissions are in fact increasing more rapidly than was expected even in the most fossil fuel intensive of the IPCC scenarios (or possible futures). This is due to rapid economic growth, especially in developing countries such as China and India. Globally, fossil fuel burning and industrial processes have increased growth rates of emissions from about 1% per annum in 1990-1999 to more than 3% per annum for 20002004. It is made worse by a growth in emissions per unit economic growth, and by increasing populations. The growth rate in emissions is greatest in developing economies such as China, although emissions are still increasing in most developed countries, which have emitted the bulk of emissions since the industrial revolution. These developments pose serious issues regarding global equity in any real attempt to reduce global emissions. There are many ways of tackling these problems including putting a price on carbon emissions, increasing energy efficiency, increased deployment of renewable technologies (Diesendorf, 2007; TREC, 2007), carbon capture and sequestration, and even nuclear
energy. All have their pros and cons, and will need to be treated individually and with respect to local circumstances. But what is clear is that the urgency of reducing emissions, say by a couple of percent per year, rather than the present increase of a few percent a year, will require urgent and large-scale action. Business as usual is not a viable option, but examining the alternatives in detail would be the subject of several more talks. Further reading Rather than give a long list of primary references I will mainly refer to readily available papers, reports and books that will provide further information and primary references. ABARE, 2006; Australian Bureau of Agricultural and Resource Economics, Technology: Its role in economic development and climate change, Research Report 06.6. See: www.abareconomics.com. Diesendorf, M., 2007: Greenhouse Solutions with sustainable Energy, University of New South Wales Press, Sydney, 413 pp. Dupont, A. and G. Pearman, 2006: Heating up the Planet: Climate Change and Security, Lowy institute Paper 12. See: www.lowyinstitute.org. Lockwood, M. and C. Frohlich, 2007: Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature, Proc. Royal Soc A, doi:10.1098/rspa.2007.1880. Hansen, J.E., 2005: A slippery slope: how much global warming constitutes “dangerous anthropogenic interference”? Climatic Change, 68, 269-279. Hansen, J.E., 2007: Scientific reticence and sea level rise, Environmental Res. Letters, 2, 024002, doi:10.1088/1748-9326/2/024002. See also: http://www.columbia.edu/~jeh1/ IPCC, 2001: Full reports from each of the Working Groups are available at http://www.ipcc.ch/ IPCC, 2007a: Intergovernmental Panel on Climate Change, 4th Assessment Report, Working Group I, The Physical Science Basis, available at http://ipccwg1.ucar.edu/wg1/wg1-report.html IPCC, 2007b: Intergovernmental Panel on Climate Change, 4th Assessment Report, Working Group II, Impacts, Adaptation and Vulnerability, available at http://www.ipcc.ch/SPM13apr07.pdf. Note that the full report will be available soon (as of 30 July 2007) at www.ipcc.ch.
IPCC, 2007c: Intergovernmental Panel on Climate Change, 4th Assessment Report, Working Group III, Mitigation of Climate Change, available at http://www.mnp.nl/ipcc/pages_media/AR4-chapters.html Pittock, A.B., 2006: Are scientists underestimating climate change? EOS (Transactions of the Amer. Geophysical Union), 87 (34), 22 August 2006). Pittock, A.B., 2007: Ten reasons why climate change may be more severe than projected. In: Sudden and Disruptive Climate Change: It Likelihood, Character and Avoidance, (M.C. McCracken, J.C. Topping and F. Moore, eds.), Earthscan Pub., London, in press. Rahmstorf, S., 2007: A semi-empirical approach to projecting future sea-level rise, Science, 315, 368-370. Rahmstorf, S., A. Cazenave, J.A. Church, J.E. Hansen, R.F. Keeling, D.E. Parker and R.C.J. Somerville, 2007: Recent climate change observations compared to projections, Science, 316, 709. Raupach, M.R., G. Marland, P. Ciais, C. Le Quere, J.G. Canadell, G. Klepper and C.B. Field,, 2007: Global and regional drivers of accelerating CO2 emissions, Proc. National Acad. of Sciences (USA), 104, 10288-10293. TREC, 2007: Clean Power from the Deserts: Trans-Mediterranean Renewable Energy Cooperation. See: http://www.TREC-EUMENA.net.