Part 2 (1/2)

Paleoclimate data identifies the impact that these missing slow feedbacks have in pus.h.i.+ng temperatures higher than expected. New research matching greenhouse-gas levels with the Earth's temperature over the last 450,000 years has established the climate sensitivity with slow feedbacks to be 6 degrees. Fifty-five million years ago, in the Arctic, temperatures were 11 degrees warmer than the ECS models would predict - which also suggests that other feedback mechanisms were at work.

A paper published in the June 2005 issue of Nature supports the theory of even higher climate sensitivity. It describes research led by Meinrat Andreae of the Max Planck Inst.i.tute for Chemistry in Germany, which used climate models and various aerosol-cooling a.s.sumptions to find the 'best fit' for the data involved in a climate sensitivity in excess of 6 degrees. By studying the planet's climate history over the last 50 years and fitting it to various climate-model options, they concluded that the effects of airborne particle pollution (or aerosols: soot and exhausts from burning fossil fuels, industrial pollution, and dust storms) and climate sensitivity are both much higher than generally a.s.sumed. They say that greater pollution controls and 'clean air' legislation will remove much of the aerosol cooling, and that if carbon dioxide levels are double their pre-industrial levels by 2100, a rise of 6 degrees can be expected. When this understanding is combined with predictions that parts of the natural carbon cycle after 2050 will reverse from being net absorbers to net emitters of carbon, they say that warming by 2100 may be as high as 10 degrees.

These findings have enormous implications. A long-term climate sensitivity of 6 degrees would mean that we have already pa.s.sed the widely advocated 2-degree threshold of dangerous anthropogenic interference with the climate. It would, therefore, require us to find the means to engineer a rapid reduction of current atmospheric greenhouse gas even to restrict global warming to below 2 degrees - a target which we believe is, in any case, far too high.

A key question is whether the slow feedbacks have started to operate. In the case of the Greenland and Antarctic ice sheets, the data is already disturbing. One of the most important slow feedbacks to be considered is the reversing of the carbon cycle - as the oceans and soils take up less carbon dioxide - and the significant amounts of methane and carbon dioxide that are released by the permafrost.

Understanding how the carbon cycle works and how changes in the cycle will affect global warming are important in understanding the scale of action required to avoid catastrophic climate changes.

The carbon cycle is the flow and exchange of carbon in its various forms (including carbon dioxide, methane, and calcium carbonate) between the planet's four large, interconnected carbon reservoirs: the atmosphere (carbon dioxide); the oceans (carbon dioxide dissolved in seawater, carbon incorporated in living and non-living plants and animals, and methane trapped under pressure on the ocean floor); fossil organic carbon (coal, gas, and oil); and the land-surface biosphere (including soils, plants, and freshwater systems). Larger amounts are stored in the Earth's crust as rock carbonates, but these are relatively immobile.

The greatest carbon reservoir is the ocean, which contains about six times the amount of carbon that is stored in plants and soils. The fossil-fuels reservoir is of similar size to the land surface biosphere, while the atmospheric sink is the smallest.

Carbon flows between these reservoirs are driven by a variety of biological, physical, and chemical processes. Examples include extracting and burning fossil fuels; animal respiration; the exchange between the atmosphere and the oceans; drawing down carbon from the atmosphere by plant photosynthesis; and destroying forests by fire, land clearing, or decomposition.

Carbon reservoirs that absorb more carbon dioxide than they emit are called carbon sinks (as opposed to carbon sources, which emit more carbon dioxide than they absorb). The ocean is a carbon dioxide sink that responds rapidly to rising levels of atmospheric carbon, but not rapidly enough to meet the present need. The ocean water soaks up some of the additional carbon dioxide, and calcifying marine organisms absorb some of it (with subsequent burial in sea-floor sediments). Forests and gra.s.slands also absorb some carbon dioxide by photosynthesis. Much of the carbon dioxide, however, remains in the atmosphere.

Many sinks governed by living organisms become less effective as the environment heats up. Though it has long been expected that the capacity of the Earth's carbon-drawdown mechanisms would decrease due to human activity and as a consequence of higher temperatures, changes already observed suggest that this is happening earlier than antic.i.p.ated. The fraction of total human-caused carbon dioxide emissions that remain in the atmosphere has increased slowly with time - which implies a slight weakening of sinks, relative to emissions.

But some sinks may get to a point where they stop drawing down carbon and start emitting it instead. In 2000, a landmark study led by Peter c.o.x, then at the UK's Hadley Centre, found that about half the present emissions are being absorbed by the ocean and by land ecosystems. But this absorption is sensitive to the climate, as well as to atmospheric carbon dioxide concentrations. These two factors are creating a feedback loop, so that, under a 'business as usual' scenario, the terrestrial biosphere will only act as an overall carbon sink until about 2050, when it will fail and revert to being a carbon source. This slow feedback will increase temperatures by another 1.5 degrees by 2100.

Research published in October 2007 by Joseph Canadell, the executive director of the Global Carbon Project, confirmed that significant contributions to the growth of atmospheric carbon dioxide arise from the slow-down in the rate of absorption of natural sinks, or from 'a decrease in the planet's ability to absorb carbon emissions due to human activity'. According to Canadell: 'Fifty years ago, for every tonne of carbon dioxide emitted, 600kg were removed by land and ocean sinks. However, in 2006, only 550kg were removed per tonne and that amount is falling.' The data suggests that from 19592006 there was an implied decline of 10 per cent in the efficiency of natural sinks. Of the recent acceleration in the rise of atmospheric carbon dioxide levels, 18 per cent is attributed to the decreased efficiency of natural sinks.

Another key factor in this decreased efficiency has been identified by Peter c.o.x, now at Britain's Centre for Ecology and Hydrology in Dorset, who says that while plants are absorbing more carbon dioxide (because photosynthesis speeds up with warming), warming also encourages plant material in the soil to break down and release carbon dioxide. A lag between these events has seen the rise in carbon dioxide levels slowed for the last two decades; but science writer Fred Pearce says, 'Soon the biosphere will start to speed it up'. According to c.o.x, a possible surge of carbon dioxide into the atmosphere in 2003 is the first evidence of this process.

c.o.x spent years researching carbon cycles while at the Hadley Centre in Exeter, which has one of the world's most highly regarded climate-modelling systems. A summary of some of the centre's modelling work, published in 2005, included two startling graphs. In one, the amount of total carbon stored in the Amazon forest and soils shows a drop from around 70 billion tonnes of carbon in 2000 to just 20 billion tonnes of carbon by 2100. The second, using the same technique, compares vegetation and soil carbon levels in 2100 to those in 1850. While vegetation carbon had increased by about 60 billion tonnes of carbon by 2100, the amount of soil carbon had decreased by 130 billion tonnes.

The Amazon hosts a quarter of the world's species, and accounts for 15 per cent of land-based photosynthesis, as well as being an engine of regional and global atmospheric circulation and regional rainfall. Yadvinder Malhi of the Environmental Change Inst.i.tute in Oxford led a team that concluded that the Amazon is warming at 0.25 degrees per decade, a rate twenty-five times faster than the temperature increase at the end of last ice age. There has already been an observed drying. Periods of recent drought in parts of the Amazon have increased the frequency of forest fires. With a total bioma.s.s store of 120 billion tonnes of carbon and predictions of large-scale drought in the eastern Amazon, the release of stored carbon by wildfires would be catastrophic.

Professor Guy Kirk of Britain's National Soil Resources Inst.i.tute calculated that since 1978, the carbon lost by Britain's soil has increased by 13 million tonnes of carbon dioxide per year - more than the 12.7 million tonnes a year that Britain saved by cleaning up its industrial emissions as part of its commitment to the Kyoto Protocol. The loss is likely to be due to plant matter and soil organic material decomposing at a faster rate as temperatures rise - an effect that is expected to compound as temperatures increase. 'It's a feedback loop,' says Kirk. 'The warmer it gets, the faster it is happening.' It is thought that the terrestrial carbon sink will begin to convert to a carbon source at an increase of 23 degrees.

Bristol University researchers also argue that the previously unexplained surge of carbon dioxide levels in the atmosphere in recent years is due to more greenhouse gas escaping from trees, plants, and soils. Global warming is making vegetation less able to absorb the carbon pollution pumped out by human activity. Wolfgang Knorr believes that 'we could be seeing the carbon cycle feedback kicking in, which is good news for scientists because it shows our models are correct. But it's bad news for everybody else'. Another bad sign comes from Canada's Manitoba region, where a study of a one-million-square-kilometre area of northern boreal forest found that the area is now releasing more greenhouse gases than it absorbs, because of an increased incidence of forest fires. This is consistent with predictions that climate change, by producing hotter and drier conditions, would lead to more fires. 'Those wildfires have caused this transition in the boreal forest from a carbon sink to a carbon source ... Climate change is what's causing the fires; if it was left unchecked, it could become a feedback,' says Tom Gower of the University of Wisconsin. A further consequence of wildfires is that more sunlight reaches the ground. This increases the rate of decomposition of organic matter, releases more carbon dioxide and, perhaps, contributes to the melting of the underlying permafrost.

Burning rainforests are also emitting hundreds of millions of tonnes of carbon dioxide each year. During the 200506 Amazon drought, thousands of square kilometres of land burned for months, releasing more than 100 million tonnes of carbon. Philip Fearnside of the National Inst.i.tute for Research in the Amazon says that 'the threat of a ”permanent El Nino” is to be taken very seriously ... Disintegration of the Amazon forest, with release of the carbon stocks in the bioma.s.s and soil, would be a significant factor in pus.h.i.+ng us into a runaway greenhouse'. Daniel Nepstad, head of the Woods Hole Research Center's Amazon program, says: [It is] not out of the question to think that half of the basin will be either cleared or severely impoverished just 20 years from now ... The nightmare scenario is one where we have a 2005-like year that extended for a couple of years, coupled with a high deforestation where we get huge areas of burning, which would produce smoke that would further reduce rainfall, worsening the cycle. A situation like this is very possible. While some climate modellers point to the end of the century for such a scenario, our own field evidence coupled with aggregated modelling suggests there could be such a dieback within two decades.

In October 2007, there were more than 10,000 points of fire across the Amazon, most of them having been set by ranchers to clear land. 'These fires are the suicide note of mankind,' says Hylton Murray-Philipson of the London-based charity Rainforest Concern.

A survey on tipping points, led by Tim Lenton of the University of East Anglia and published in early 2008, found that leading researchers estimated that there was a medium risk that the Amazon would be largely destroyed by 2050. (Regarding other potential tipping points, they also estimated a medium risk of the Indian summer monsoon destabilising within one year; the West African monsoon collapsing in 10 years; and the Arctic boreal forest dying in 50 years.) Total carbon emissions from tropical deforestation are estimated at 1.5 billion tonnes of carbon a year, including illegal fires in Indonesia's vast peatlands, the haze from which regularly blankets Sumatra and Malaysia. Indonesia's peat swamps contain 21 per cent of the Earth's land-based carbon, and are now subject to increasing clearing, drying, and burning. During the 1997 El Nino event, an estimated 0.81 2.57 billion tonnes of carbon was released to the atmosphere as a result of burning peat and vegetation in Indonesia. This is equivalent to 1340 per cent of the mean annual global-carbon emissions from fossil fuels. This burning also contributed greatly to the largest annual increase in atmospheric carbon dioxide concentration ever detected since records began in 1957.

New a.n.a.lysis of two decades of data from more than 30 sites also indicates that the ability of forests in the frozen north to soak up man-made carbon dioxide is weakening.

The melting of permafrost (permanently frozen soil, or soil below the freezing point of water) is another 'slow' feedback that is adding to global warming. As the Arctic warms, permafrost in the northern boreal forests, and further north in the Arctic tundra, is starting to melt. As it melts, its thick layers of thawing peat trigger the release of methane and carbon dioxide, both greenhouse gases.

With less than 1 degree of warming, Arctic ground that has been frozen for 3000 years is melting and producing thermokarst (a land surface that forms as ice-rich permafrost melts). Even under scenarios of modest climate warming, this could affect 1030 per cent of Arctic lowland landscapes, and severely alter tundra ecosystems. As the permafrost thaws, lakes form and microbes convert the soil's organic matter into methane. The methane bubbles through the surface water into the atmosphere. In dry conditions, the warming soil also releases carbon dioxide.

A 2006 study found that Siberia's thawing wetlands are a significant, underestimated source of atmospheric methane, with lakes in the region growing in number and size, and emission rates appearing to be five times higher than previously estimated. The NCAR in Boulder predicts that half of the permafrost will thaw to a depth of 3 metres by 2050. As glaciologist Ted Scambos says: 'that's a serious runaway ... a catastrophe lies buried under the permafrost.'

The western Siberian peat bog is amongst the fastest-warming places on the planet, and Sergei Kirpotin of Tomsk State University calls the melting of frozen bogs an 'ecological landslide that is probably irreversible'. One estimate puts methane releases from the current area of melting bog at 100,000 tonnes per day.

Russian Arctic climate researcher Sergei Zimov frames the gravity of the situation well: 'Permafrost areas hold 500 billion tonnes of carbon, which can fast turn into greenhouse gases ... The deposits of organic matter in these soils are so gigantic that they dwarf global oil reserves ... If you don't stop emissions of greenhouse gases into the atmosphere ... the Kyoto Protocol will seem like childish prattle.'

The ocean carbon-cycle feedback is also a significant slow-feedback contributor. Part of the decline in sink capacity comes from a decrease of up to 30 per cent in the efficiency of the Southern Ocean sink over the last 20 years. This decrease has been attributed to the strengthening of the winds around Antarctica, which enhances ventilation of natural, carbon-rich deep waters. Lead author Corinne Le Quere of the University of East Anglia says: This is the first time that we've been able to say that climate change itself is responsible for the saturation of the Southern Ocean sink. This is serious. All climate models predict that this kind of 'feedback' will continue and intensify during this century. The Earth's carbon sinks - of which the Southern Ocean accounts for 15 per cent - absorb about half of all human carbon emissions. With the Southern Ocean reaching its saturation point, more carbon dioxide will stay in our atmosphere.

This finding follows pioneering work by CSIRO marine research scientists, including Stephen Rintoul and John Church, that seeks to understand how the Southern Ocean influences the climate system, its patterns of circulation, and the region's role in the global ocean-circulation system.

Measurements of the North Atlantic taken between the mid-1990s and 2005 found that, in the course of that decade, the amount of carbon dioxide in the water had reduced by half. It is suggested that warmer surface water was reducing the amount of carbon dioxide being carried down into the deep ocean. Lead researcher, Andrew Watson of the University of East Anglia, concludes: 'We suspect that it is climatically driven, that the sink is much more sensitive to changes in climate than we expected ... if you have a series of relatively warm winters, the ocean surface doesn't cool quite so much ... so the carbon dioxide is not being taken down into the deep water'. He warned that the process may fuel climate change: 'It will be a positive feedback, because if the oceans take up less carbon dioxide then carbon dioxide will go up faster in the atmosphere and that will increase the global warming.'

Satellite data gathered over the past ten years shows that the growth of marine algae, the basis of the entire ocean food chain, is being affected adversely by rising sea temperatures. Algae, the microscopic plants that permeate the oceans, remove up to 50 billion tonnes of carbon dioxide per year from the Earth's atmosphere. This system is as effective in removing carbon dioxide from the air as all plant life on the planet's land surface.

Jeff Polovina of Hawaii's National Marine Fisheries Service laboratory says that satellite imagery shows that green colouration (indicating chlorophyll life) in the middle of the ocean is fading away: 'The regions that are showing the lowest amount of plant life, which [are] sometimes referred to as the biological deserts of the ocean, are growing at roughly 1 to 4 per cent per year.' While such areas expanding are consistent with global warming scenarios, the rates of expansion already observed greatly exceed recent model predictions.

Increasing ocean acidification will also weaken marine life. This occurs as some of the carbon dioxide absorbed by the ocean reacts with water molecules to produce carbonic acid, which lowers the ocean's pH. The oceans are already 30 per cent more acidic than they were at the beginning of the Industrial Revolution, more than two centuries ago. If emissions continue at 'business as usual' rates, carbon dioxide levels in the oceans will rise so high that, by 2050, the ocean will be so acidic that current US water-quality standards would have to categorise it as industrial waste. Stanford University chemical oceanographer Ken Caldeira states that, if unabated, this could potentially cause the extinction of many marine species: 'What we're doing in the next decade will affect our oceans for millions of years ... carbon dioxide levels are going up extremely rapidly, and it's overwhelming our marine systems.'

Waters around the Great Barrier Reef are also acidifying at a higher-than-expected rate. Ecosystem collapse caused by acidification will likely reduce marine bioma.s.s and, therefore, the capacity of the oceans to absorb carbon dioxide. Professor Malcolm McCulloch of the Australian National University says that, contrary to previous predictions, this acidification is now taking place over decades, rather than centuries: '[T]he new data on the Great Barrier Reef suggests the effects are even greater than forecast.'

Acc.u.mulating evidence suggests that slow feedbacks from oceans, soils, and permafrost are already affecting the climate system.

CHAPTER 6.

Most Species, Most Ecosystems.

Martin Parry, co-chairman of one of the three IPCC working groups, told his audience at the launch of the full 2007 IPCC report on the impacts of global warming: 'We are all used to talking about these impacts coming in the lifetimes of our children and grandchildren. Now we know that it's us.' He said that destructive changes in temperature, rainfall, and agriculture were now forecast to occur several decades earlier than expected - and that means a huge threat to biodiversity.

As global temperatures rise, many species have to migrate towards the poles to stay in their habitable zones. If they can't migrate at sufficient speed, many species will be lost, and many ecosystems will degrade. During rapid change, such as the deglaciation and warming that occurred after the last ice age about 15,000 years ago, some widespread and dominant species became extinct when temperatures rose 5 degrees over a span of 5000 years. That is a rate of increase of 0.01 degrees per decade - 20 times slower than today's rate of change.

Cagan Sekercioglu from Stanford University says that the IPCC's worst-case scenario to 2100, combined with extensive habitat loss, would result in the extinction of around 30 per cent of land bird species. With warming, birds will try to move to higher alt.i.tudes. Once the top of a mountain is reached, there is nowhere left to go. In the lowland tropics, where most bird species live, there can be no significantly higher slopes to which they can retreat.

The rate of change in temperature is also very important in determining the impact it will have, because many ecosystems and species are sensitive to small temperature changes. A study by Rik Leemans and Bas Eickhout found that if a 2-degree impact builds up slowly over 1000 years, most affected ecosystems are likely to adapt (most often by moving); but if the same rise happens in 50 years (0.4 degrees per decade), many ecosystems will deteriorate rapidly.

At 0.4 degrees per decade, the isotherms (bands of equal temperatures) will be moving towards the poles at about 120 kilometres per decade; at this rate of temperature change, most ecosystems will be torn apart. Interestingly, Australia's birds are moving south at a rate of 100150 kilometres a decade with only half this rate of warming. Very fast-moving species will migrate with the temperature changes if they can survive in the ecosystems into which they move. Slow-moving species will not be able to keep up with the movement of their preferred temperature band and, unless they are tolerant of high temperatures and not dependent on species that have moved on, they will die out. At 0.4 degrees of change per decade, the isotherms are moving so fast that virtually all ecosystems will not be able to survive, and very large percentages of the dependent species will die out; yet this is the rate antic.i.p.ated in some of the IPCC scenarios by mid-century, and few scenarios antic.i.p.ate rates of less than 0.3 degrees per decade.

A 2007 study of the IPCC report's low- and high-emission scenarios, led by Dian Seidel of the NOAA in Was.h.i.+ngton, found that up to 39 and 48 per cent, respectively, of the Earth's terrestrial surface may experience novel and disappearing climates by 2100. Work published two years earlier projected the effects on 1350 European plant species under seven climate-change scenarios, and found that more than half could be vulnerable or threatened by 2080. The risk of extinction for European plants may be large, even in moderate scenarios of climate change.

Over the past 25 years, the area defined as 'climatologically tropical' has expanded to the north and south, away from the equator by about 2.5 degrees of lat.i.tude in each direction. This is equivalent to a rate of 110 kilometres per decade, and is greater than the IPCC's worst-case scenario of a total predicted s.h.i.+ft of 2 degrees of lat.i.tude by 2100. This will disrupt the tropicaltemperate geographic transition of ecosystems and, if maintained over a century timescale, it suggests that few of the affected ecosystems would adapt at the implied warming of greater than 0.3 degrees per decade.