2010 Diary week 46
Climate Change
Book Review
Below you will find Part 5 of the review of Climate Change: Turning up the Heat by A. Barrie Pittock. These are some snippets: “Human-induced climate change is only an issue if it is large enough and rapid enough to create real problems for natural ecosystems and for human societies. In this and the following chapter we will look at the magnitude and rate of climate change, including sea-level rise and changes in extreme events, that are likely to result from human-induced emissions of greenhouse gases, and at what the effects might be on nature and society.” “The IPCC’s 2001 report projects global average surface temperature increases from 1.4 to 5.8°C by 2100, compared to the 1992 range of 1.5 to 3.5°C, or roughly 2 to 10 times that observed during the 20th century.” “Estimates by David Stainforth of Oxford University place the range from 1.9 to 11.5°C. This would extend the range of possible warmings upwards, but the new results have yet to be fully considered.” “More changes in extreme events are likely in the 21st century. Daily maximum and minimum temperatures, and the number of hot days are very likely to increase, with fewer cold and frosty days. The heat or discomfort index is very likely to increase in most tropical and mid-latitude areas. More intense precipitation events are very likely over many areas (causing more frequent flooding), and increased drying is likely over mid-latitude continental interiors, with an increased risk of drought.”
CLIMATE CHANGE
TURNING UP THE HEAT
A. BARRIE PITTOCK
EARTHSCA/CSIRO PUBLISHING 2005
PART V
Chapter 5: What Climate Changes Are Likely?
• Human-induced climate change is only an issue if it is large enough and rapid enough to create real problems for natural ecosystems and for human societies. In this and the following chapter we will look at the magnitude and rate of climate change, including sea-level rise and changes in extreme events, that are likely to result from human-induced emissions of greenhouse gases, and at what the effects might be on nature and society.
• Complete surprises are possible. A prime example is the sudden appearance of the ‘ozone hole’, which first occurred over Antarctica during the 1970s in the Southern Hemisphere’s spring.
• ‘Repairing’ the ozone hole is likely to take the best part of a century, despite strong international agreements on doing so (see Chapter 9).
Projected climate changes
• The magnitude of eventual climate change depends to a first approximation on the accumulated emissions.
• The projected accumulated emissions by 2100, expressed in units of thousands of millions of tonnes of carbon equivalent (GtC), range from 770 GtC to 2540 GtC, corresponding to 540 parts per million (ppm) to 970 ppm.
• Likely changes included surface warming, changes to rain and snowfall, increased evaporation, changes to the magnitude and frequency of extreme events such as droughts and floods, rises in sea level, and the possibilities of abrupt changes and large-scale changes of the climate system such as ocean currents or the melting of ice sheets.
Surface warming
• The IPCC’s 2001 report projects global average surface temperature increases from 1.4 to 5.8°C by 2100, compared to the 1992 range of 1.5 to 3.5°C, or roughly 2 to 10 times that observed during the 20th century.
• The rate is much faster than the average warming at the end of the last glaciation.
• Estimates by David Stainforth of Oxford University place the range from 1.9 to 11.5°C. This would extend the range of possible warmings upwards, but the new results have yet to be fully considered.
• Because of the slow response of the climate system, the warming at 2100 is largely determined by changes already in the pipeline due to past emissions.
Regional warmings
• Warming in continental interiors and in high latitudes of the northern hemisphere is expected to be greatest with less expected over the Southern Ocean due to its large capacity to transport surface heat into the deep ocean, and possibly in the North Atlantic region, depending on the behaviour of the ocean circulation (see later).
• Warming may be greater in the eastern tropical Pacific than in the west, which may lead to a more El-Niňo-like average condition.
• After stabilization of greenhouse gas concentrations in the atmosphere, warming will continue for centuries in the Southern Ocean, leading to ongoing regional climate change in the vicinity, especially in Australia and New Zealand, and possibly in southern Argentina and Chile.
Precipitation and evaporation
• Global average precipitation (rain or snowfall) and evaporation are projected to increase by about 1 to 9% by 2100, depending on which scenario and climate model is used.
• Projected precipitation changes vary more from region to region, with increases over northern mid-to high latitudes and Antarctica in winter. At lower latitudes there are both regional increases and decreases over land areas.
• In the northern winter, rainfall is projected to increase in tropical Africa, show little change in South-east Asia and decrease in Central America.
• An increase or little change is expected in South Asia in the northern summer, but a decrease is expected in the Mediterranean region. Winter rainfall is expected to decrease over Australia.
• Rainfall changes will vary greatly on finer spatial scales due to topographic and coastal effects.
• Changes in rainfall intensity and seasonality are expected, but these changes are quite uncertain in many areas.
• Higher temperatures will mean that more precipitation will fall as rain rather than snow, changing the seasonality of river flows in many snow-fed catchments such as those in the western United States and northern Europe.
• There has been an observed strengthening and poleward movement of the atmospheric low-pressure belts around the North and South Poles during the late 20th century. This effect is expected to continue through the 21st century. Storm belts will also shift, affecting climate over Europe.
Extreme events
• More changes in extreme events are likely in the 21st century. Daily maximum and minimum temperatures, and the number of hot days are very likely to increase, with fewer cold and frosty days.
• The heat or discomfort index is very likely to increase in most tropical and mid-latitude areas.
• More intense precipitation events are very likely over many areas (causing more frequent flooding), and increased drying is likely over mid-latitude continental interiors, with an increased risk of drought.
• The intensity of tropical cyclone winds and peak rainfalls is likely to increase. Greater extremes of flood and drought are likely with the El Niňo-Southern Oscillation cycle (El Niňo and La Niňa), which is expected to continue. These changes are summarized in Table 4.
• A review of the paleo-records of natural floods found the magnitude and frequency of floods are highly sensitive to even modest changes of climate equivalent to or smaller than those expected from global warming in the 21st century.
• Times of rapid climate change have a tendency to be associated with more frequent occurrences of large and extreme floods.
• A study of great flood events in 29 river basins around the world shows an increase in frequency since 1953. The study finds that the frequency of such large floods with increase further during the 21st century, by a factor 2 to 8.
• An increase in intense precipitation is very likely in many parts of Europe, despite a possible reduction in summer rainfall over a large part of the continent. Thus, severe flooding may become more frequent, despite a general tendency toward drier summers.
• The proportion of total precipitation derived from extreme and heavy events will continue to increase relative to that from light to moderate events.
• Large floods and widespread droughts are commonly due to one or other extreme of naturally occurring variations in circulation patterns including the North Atlantic Oscillation (NAO) and the El Niňo-Southern Oscillation (ENSO).
• A progressive shift in the NAO toward its more positive phase has been observed since 1950, and has been associated with a slow warming of the tropical oceans.
• Future behaviour of ENSO is of critical importance to many countries affected by ENSO-related rainfall variations and storm frequency.
• The IPCC concluded in its report in 2001 that it is likely that tropical cyclones will become more intense by 5-10% around 2050, with corresponding rainfall peak intensities increasing by about 25%.
• A statistically significant increase in explosively developing cyclones in the Southern Hemisphere has been detected from 1979 to 1999.
• Global warming could also influence another class of extreme event that was not considered by the IPCC in its 2001 report, namely earthquakes.
• The weight of glaciers and ice sheets depresses the Earth’s crust under them, and their removal leads to the underlying crust slowly rising. There is good evidence that regional earthquake fault instability increased at the end of the last glaciation.
• Other extreme climate-related events include the occurrence of flash flooding, severe thunderstorms and hail, landslides, extreme sea-level events and wildfires. Some of these will be discussed further in Chapter 6 on impacts.
Sea-level rise
• Sea-level rise is obviously important, given that a rapidly increasing number of people live in low-lying coastal areas. Projections of global sea-level rise by the IPCC in 2001 range from 9 to 88 cm by 2100.
• A large part of sea-level rise in the next 100 years is determined by global warming to date, since there is a large delay in its effect on sea level.
• The main contribution to sea-level rise will be from thermal expansion of sea water, which will add some 11 to 43 cm, accelerating through the 21st century, with the next largest contribution coming from the melting of mountain glaciers (1 to 23 cm).
• Melting from Greenland is likely to add little to sea-level rise (-2 to +9 cm),while Antarctica may make a negative contribution due to increased snow accumulation on the ice cap (-17 to +2 cm).
• Antarctica’s contribution to sea-level changes is particularly uncertain, with the possibility of surprises. The Wordie and Larsen A and B shelves broke up very rapidly during the 1990s and early 2000s following regional warming of about 2.5°C over the previous 50 years.
• The 1600 square kilometers of Larsen A suddenly disintegrated in 39 days during 1994-95, and the 3245 square kilometers of Larsen B in only 41 days in 2002. The very rapid disintegration of the Wordie and Larsen Ice Shelves was unexpected.
• The rapid disintegration of the Wordie and Larsen Ice Shelves should perhaps cause a rethink as to the stability of the West Antarctic Ice Sheet (WAIS), which, if it were to totally disintegrate, would add some 6 metres to global average sea level.
• Recent aircraft and satellite observations of the Amundsen Sea sector of West Antarctica show that local glaciers are discharging about 250 cubic kilometers of ice each year to the ocean, almost 60% more than is accumulating in their catchment areas, and at a faster rate than in the 1990s.
• There is a similar concern about more rapid disintegration of the Greenland ice sheet, which if it were to melt would cause about a 7 metre rise in sea level.
• Sea-level changes will not be uniform across the world. Extreme high levels due to storm surges will occur with increasing frequency as a result of average sea-level rise.
Abrupt changes, threshold and instabilities
• Large-scale abrupt changes in the climate system have already been discussed in Chapter 2. The problem for climate change projections is to capture the possibility and the probability of, and risk from, these abrupt changes.
• The IPCC report in 2001 emphasised the potential importance of plausible or irreversible Earth system events, when it stated:
Human-induced climate change has the potential to trigger large-scale changes in Earth systems that could have severe consequences at regional or global scales. The probabilities of triggering such events are poorly understood but should not be ignored, given the severity of the consequences.
• Surface waters in the North Atlantic Ocean presently become colder and more saline as they travel north-east in the Gulf-Stream. This leads to convective overturning (sinking) of the surface waters, which helps drive the currents.
• The slow-down or complete cessation of convective overturning of the waters of the North Atlantic could arise due to several causes, including surface warming due to the enhanced greenhouse effect, and lower salinity of surface waters due to increased rainfall at high latitudes and influxes of freshwater from rivers, melting glaciers and partial melting of the Greenland ice sheet.
• Large-scale salinity changes have been observed in the world’s oceans, especially the Atlantic.
• There is clear evidence of strengthening of the hydrological cycle, with increased evaporation at low latitudes and increased precipitation at high latitudes. A continuation of this trend could trigger a cessation of overturning.
• Around Antarctica the main cause of convective overturning of the surface ocean is the freezing of seawater, which leads to a rejection of salt, and thus to dense highly saline water that sinks. Global warming would lead to a reduction in sea-ice formation, and thus reduced overturning there as well.
• Besides direct impact on surface climate, reduced overturning of the oceans would reduce oceanic uptake of carbon dioxide, and thus further increase the carbon dioxide concentration in the atmosphere.
• Melting of the Greenland ice sheet and/or disintegration of the WAIS, both of which could be triggered by global warming but would take centuries to complete, are potentially irreversible.
• These events are probably inevitable if carbon dioxide concentrations are allowed to reach two to three times pre-industrial values. The process could only be reversed if carbon dioxide were to be taken out of the atmosphere in such large quantities as to substantially reduce the carbon dioxide concentration and thus reverse global warming.
• Failing this, the Greenland and West Atlantic ice sheets would each contribute several metres to global mean sea level over the next thousand years or so.
• Time scales for disintegration of the WAIS and the Greenland ice sheet are still under debate and there is some chance that they may occur more rapidly. Rapid melting of the Greenland ice sheet could also affect the ocean circulation.
• Several mechanisms exist which could lead to an acceleration of global warming via positive feedbacks (amplification mechanisms) associated with the carbon cycle, a process known as ‘runaway carbon dynamics’.
• One is the destabilization of the huge methane reserves stored in crystalline structures (hydrates) on the seabed of the continental shelves and slopes, and in permafrost regions on land.
• In Arctic coastal areas sea ice is receding and increased wave action is already accelerating erosion of shorelines.
• Melting of permafrost and loss of structural integrity of hydrate deposits will increase slope failures, landslides and avalanches and threaten human infrastructure such as buildings, roads and pipelines.
• In a 2002 review, Euan Nisbet of the University of London, concluded that the case is open for a major methane release in the 21st century possibly from a large pool of free methane gas trapped below a hydrate deposit.
• Other possible feedbacks on the carbon cycle include decreased efficiency of the ocean and terrestrial biospheric sinks of carbon due to global warming.
• The terrestrial biosphere, which presently acts as a sink of carbon dioxide, could become a source by 2050, and the ocean sink may also be reduced due to greater stratification of the surface layers of the ocean.
• Widespread forest and peat fires were observed to contribute significantly to carbon dioxide increases in 1994/95 and 1997/98 when large biomass burning took place in tropical and boreal regions.
• The IPCC report discusses the possibilities of major changes in the behaviour of the continental monsoons, the ENSO and other patterns of climate variability.
Scenarios in a nutshell
• In summary, the full range of IPCC SRES emissions scenarios would lead to a wide range of global warmings (1.4 to 5.8°C) and sea-level rise (9 to 88 cm) by 2100.
• Emissions policies that aim at stabilizing greenhouse gas concentrations in the atmosphere anywhere below 1000 ppm of carbon dioxide equivalent would lead to smaller warmings than those resulting from the higher SRES scenarios.
• A target concentration of 450 ppm at stabilization would lead to an estimated warming by 2100 in the range of 1.2 to 2.3°C, while a target of 1000 ppm would lad to 2.0 to 3.5°C
• However, warmings and sea-level rise would continue long past 2100, even in stabilization scenarios.
• Changed frequencies and intensities of extreme weather events are likely with global warming, including more hot days, fewer cold nights, greater heat stress, more droughts in mid-latitude continental areas, more intense rain events, and increased intensity and rainfall from tropical cyclones or hurricanes.
• Possibilities exist for sudden rapid and long-term changes in global-scale climate-related systems, including more rapid sea-level rise due to more rapid disintegration and melting of the Greenland and West Antarctic ice sheets, major changes to the circulation of the oceans with regional climate impacts (especially in regions bordering the North Atlantic), and accelerated release of methane and carbon dioxide into the atmosphere which would accelerate global warming.
• The possibility that climate sensitivity may be higher than the range adopted by the IPCC in 2001 raises the possibility of climate changes greater than those projected in the 2001 report. New estimates by a UK team of scientists in 2005 put the climate sensitivity range at 2 to 11°C. This would mean a much greater chance of more severe climate changes and impacts.
• Even the projected global warmings reported by the IPCC in 2001 lead to climate-related changes that would have major impacts on human and natural systems, as we shall see in the next chapter.
• This means that the stakes are high, and suggests there is a need for reducing greenhouse gas emissions so we can follow low emissions pathways. How this might be achieved is the subject taken up later in the book.