All the impacts discussed above assume that there is a linear relationship between greenhouse gas forcing and climate change, as produced by the AOGCMs. There is, however, increasing concern among scientists that climate change may occur abruptly. This is because there is recent scientific evidence that many past climatic changes have occurred with startling speed. For example, ice-core records suggest that half the warming in Greenland since the last ice age was achieved in only a decade. Some of these regional changes involved temperature changes of over 10°C. This relates back to Chapter 1 and the discussion of how climate changes, whether it varies smoothly or contains thresholds and bifurcations. Such is the concern that future climate change may be abrupt that in 2003 the prestigious Royal Society in London convened a conference and an associated report on this very topic, while the National Research Council in the USA commissioned a report on Abrupt Climate Change, published in 2002. Though this is a new paradigm, the ability of the global climate system to change abruptly has been well established by research over the last decade. What both reports stress is the need for the wider community of natural and social scientists, as well as policy makers, to recognize this new paradigm and act accordingly. The National Research Council (NRC) Report makes five recommendations:
1. Improve the fundamental knowledge base related to abrupt climate change.
Below I review three possible abrupt climate surprises: deep-ocean circulation, gas hydrates, and Amazonia. But what connects them all is that we really do not know how the global climate will react to global warming in the future. It is thus essential for more work to be done on how abruptly these changes occur. Moreover, the NRC report suggests there is need for more understanding of how the global and regional economies would deal with abrupt climate change.
2. Improve modelling focused on abrupt climate change.
At the moment most models try to achieve a steady-state or equilibrium between the forcing and the variations. What is required is a new type of high-resolution model to look at how easily ^ abrupt climate change can occur. The NRC report stressed that new J possible mechanisms of abrupt climate change should be $
investigated and a hierarchy of models will be required, since many of these abrupt changes are initiated at the fine spatial scale, which AOGCMs are currently unable to simulate.
3. Improve palaeoclimatic data related to abrupt climate change.
Past climate changes have provided us with many of the clues about how future climate could change. For example, oceanographers had not considered the idea that the deep-ocean circulation could change until it was shown that it was radically different during the last ice age. The NRC report suggests that improvement is required in both geographical and temporal resolution of abrupt events in the past. Also there is a need to focus on water, both too much (floods) and too little (droughts), as these are by far the most important influences on humanity.
This has been mentioned before in this book, but current practice in climate statistics is to assume a simple unchanging distribution of outcomes. For example, a one-in-30-year storm will statistically always occur once in 30 years. This assumption leads to serious underestimation of the likelihood of extreme events; hence the conceptual basis and application of climate statistics should be re-examined, particularly as all future predictions are that the year-to-year variability in extreme weather will increase in the future.
5. Investigate 'no-regrets' strategies to reduce vulnerability.
The NRC report stresses that research should be undertaken to identify 'no-regrets' measures to reduce vulnerabilities and increase adaptive capacity at little or no cost. No-regrets measures may include low-cost steps to slow climate change, improve climate forecasting, slow biodiversity loss, improve water, land, and air og quality. Technological changes, such as clean technology, may | increase the adaptability and resiliency of both economic and j| ecological systems faced with abrupt climate change. The report 3 stresses the need for research into how poor countries can be assisted to develop a more adaptable scientific and economic infrastructure to reduce the effects of abrupt climate change.
Below, I discuss just three possible 'surprises' that could occur in the next hundred years because of global warming. What is common to all these hypotheses is that we really have no idea if and when they will happen and, if they do, what will be the effects.
The circulation of the ocean is one of the major controls on our global climate. In fact, the deep ocean is the only candidate for driving and sustaining internal long-term climate change (of hundreds to thousands of years) because of its volume, heat capacity, and inertia. In the North Atlantic, the north-east trending Gulf Stream carries warm and salty surface water from the Gulf of
Mexico up to the Nordic seas (Figure 28). The increased saltiness or salinity in the Gulf Stream is due to the huge amount of evaporation that occurs in the Caribbean, which removes moisture from the surface waters and concentrates the salts in the seawater. As the Gulf Stream flows northward it cools down. The combination of a high salt content and low temperature makes the surface water heavier or denser. Hence, when it reaches the relatively fresh oceans north of Iceland, the surface water has cooled sufficiently to become dense enough to sink into the deep ocean. The 'pull' exerted by the sinking of this dense water mass helps maintain the strength of the warm Gulf Stream, ensuring a current of warm tropical water flowing into the north-east Atlantic, sending mild air masses across to the European continent. It has been calculated that the Gulf Stream delivers 27,000 times the energy of all of Britain's power stations put together. If you are in any doubt about how good the Gulf Stream is for the European climate, compare the winters at the same latitude on either side of the Atlantic Ocean, for example, London with Labrador or Lisbon with New York. Or a better comparison is between Western Europe and the West coast of North America, which have a similar geographical relationship between the ocean and continent. So think of Alaska and Scotland which are at about the same latitude.
The newly formed deep water sinks to a depth of between 2,000 and 3,500 m in the ocean and flows southward down the Atlantic Ocean, as the North Atlantic Deep Water (NADW). In the South Atlantic Ocean it meets a second type of deep water, which is formed in the Southern Ocean and is called the Antarctic Bottom Water (AABW). This is formed in a different way to NADW. Antarctica is surrounded by sea ice and deep water forms in coast polnyas or large holes in the sea ice. Out-blowing Antarctic winds push sea ice away from the continental edge to produce these holes. The winds are so cold that they super-cool the exposed surface waters. This leads to more sea-ice formation and salt rejection, producing the coldest and saltiest water in the world. AABW flows around the
Antarctic and penetrates the North Atlantic, flowing under the warmer and thus somewhat lighter NADW (Figure 29«). The AABW also flows into both the Indian and Pacific Oceans.
This balance between the NADW and AABW is extremely important in maintaining our present climate, as not only does it keep the Gulf Stream flowing past Europe but it maintains the right amount of heat exchange between the northern and southern hemispheres. Scientists have shown that the circulation of deep water can be weakened or 'switched off if there is enough input of fresh water to make the surface water too light to sink. This evidence has come from both computer models and the study of past climates. Scientists have coined the phrase 'dedensification' to mean the removal of density by adding fresh water and/or warming up the water, both of which prevent seawater from being dense enough to sink. As we have seen, there is already concern that global warming will cause significant melting of the polar ice caps. This ^ will lead to more fresh water being added to the polar oceans. J
Global warming could, therefore, cause the collapse of NADW, and $ a weakening of the warm Gulf Stream (Figure 296). This would cause much colder European winters, stormier conditions, and more severe weather. However, the influence of the warm Gulf Stream is mainly in the winter so it does not affect summer temperatures. So, if the Gulf Stream fails, global warming would still cause European summers to heat up. Europe would end up with extreme seasonal weather.
A counter scenario is that if the Antarctic ice sheet starts to melt significantly before the Greenland and Arctic ice, things could be very different. If enough melt-water is put in the Southern Ocean then AABW will be severely curtailed. Because the deep-water system is a balancing act between NADW and AABW, if AABW is reduced then the NADW will increase and expand (Figure 29c). The problem is that NADW is warmer than AABW, and because if you heat up a liquid it expands, the NADW will take up more space. So any increase in NADW will mean an increase in sea level. Computer
29. Different possible circulation of the deep ocean depending sea surface salinity (SSS), i.e., freshwater input models by Professor Seidov (Penn State University, USA) and myself have suggested that a melt-water event in the Southern Ocean could cause a reduction in the AABW and the expansion of the NADW, and would result in an average sea-level increase of 2.5 m (Figure 30). The problem is that we have no idea how much fresh water it will take to shut off either the NADW or the AABW. Nor at the moment can we predict which will melt first, the Arctic or Antarctic. We do know that these events have happened frequently in the past and have drastically altered the global climate. If global warming continues, some time in the future enough melt-water will be generated and the options will be either severe alteration of the European climate or an additional 2.5 m of global sea-level rise.
Not only do we not know how much fresh water is required to reduce either North Atlantic or Southern Ocean deep-water formation, we are also not sure whether it could be reversed. This is because computer models suggest the freshwater-deep-ocean system could be a threshold-bifurcated system. Figure 31 demonstrates this bifurcation of the climate system and shows that there can be different relationships between climate and the forcing mechanism, depending on the direction of the threshold. The bifurcation system is very common in natural systems, for example, in cases where inertia or the shift between different states of matter need to be overcome. Figure 31 shows that in cases A and B the system is reversible, but in case C it is not. In case C the control variable must increase to more than it was in the previous equilibrium state to get over the threshold and return the system to its pre-threshold state. Let us consider this in terms of the salinity of the North Atlantic versus the production of North Atlantic Deep Water (NADW). We know that adding more fresh water to the North Atlantic hampers the production of salty cold, and hence heavy deep water. In case A, changing the salinity of the North Atlantic has no effect on the amount of NADW produced. It is a very insensitive system. In case B, reducing the salinity reduces the production of NADW; however, if the salt is replaced, then the
30. Future sea-level changes depending on meltwater input in either a) Southern Ocean around Antartica or b) North Atlantic Ocean
31. Bifurcation of the climate system salinity production of NADW returns to its previous, pre-threshold level. In case C, reducing the North Atlantic salinity reduces the production of NADW. However, simply returning the same amount of salt does not return the NADW production to the normal level. Because of the bifurcation, a lot more salt has to be injected to bring back the NADW production to its previous level (see Figures 5e and 29c). It may be that the extra amount of salt required is not possible within the system and so this makes the system theoretically irreversible. The major problems we face when looking at future climate change is whether a bifurcation system exists and whether the system will go beyond a point of being reversible. What is worrying is that these threshold systems can apply to any part of the climate system. Another example is the position of the monsoons: in Oman and other parts of Arabia fresh groundwater has been dated to 18,000 years ago, to the last ice age; none of it is any younger. This suggests that under glacial conditions the modern South-East Asian monsoon belt came much further north, producing significant rains in what are now extremely arid regions. As soon as the global climate moved into an interglacial the monsoons shifted. The next question is: if global warming changes the position of the monsoons again, will they return to the present position if the effects of global warming lessen?
| Currently, below the world's oceans and permafrost lurks a deadly j| threat - gas hydrates. These are a mixture of water and methane,
3 which is sustained as a solid at very low temperatures and very high pressures. These gas hydrates are a solid composed of a cage of water molecules, which hold individual molecules of methane and other gases. The methane comes from decaying organic matter found deep in ocean sediments and in soils beneath permafrost. These gas hydrate reservoirs are extremely unstable, as a slight increase in temperature or decrease in pressure can cause them to destabilize and thus pose a major risk. The impacts of global warming include the heating up of both the oceans and the permafrost, which could cause the gas hydrates to break down, pumping out huge amounts of methane into the atmosphere. Methane is a very strong greenhouse gas, 21 times more powerful than carbon dioxide. If enough were released it would raise temperatures even more, releasing even more gas hydrates -producing a runaway greenhouse effect. There are 10,000 gigatonnes of gas hydrates stored beneath our feet compared with only 180 gigatonnes of carbon dioxide currently in the atmosphere.
The reason why scientists are so worried about this is because there is evidence that a runaway greenhouse effect occurred 55 million years ago. During this hot-house event 1,200 gigatonnes of gas hydrates were released, but it accelerated the natural greenhouse effect, producing an extra 5°C of warming. Scientists are, however, divided on whether global warming will cause a significant release of gas hydrates. The reason is that there are two controls on oceanic gas hydrates: one is temperature and the other is pressure or sea level. However, model calculations by Peter Cox at the Met Office Hadley Centre suggest that at current predicted rates of global warming, sea level will not rise faster to counter the effects of the warming ocean, hence gas hydrates will start to break down in the next hundred years, releasing methane.
There is another problem. If significant parts of the Greenland and Antarctica Ice Sheets melt, the removal of ice from the continent means that it will recover and start to move upwards. This isostatic ^ rebound can be seen in the British Isles, which are still recovering J from the last ice age, with Scotland still rising while England is $ lowering. This will mean that the relative sea level around the continental shelf will fall, removing the weight and thus the pressure of the sea water on the marine sediment. Pressure removal is a much more efficient way of destabilizing gas hydrates than temperature increases and so huge amounts of methane could be released from around the Arctic and Antarctic. There is another secondary effect of gas hydrate release: when the hydrates break down they can do so explosively. There is clear evidence in the past that violent gas hydrate releases have caused massive slumping of the continental shelf and associated tsunamis (giant waves). The most famous is the Norwegian Storegga slide which occurred about 8,000 years ago, was the size of Wales, and produced a 15 m high tsunami, which wiped out many prehistoric settlements in Scotland. Hence, we cannot rule out the fact that global warming could lead to an increased frequency of gas hydrate-generated submarine landslides and thus tsunamis of over 15 m in height hitting our coasts. Up to now, only the countries around the Pacific rim are prepared for this type of event as many of these tsunamis are set off by earthquakes. But gas hydrate-generated tsunamis could occur anywhere in the ocean.
In 1542 Francisco de Orellana led the first European voyage down the Amazon River. During this intrepid voyage the expedition met a lot of resistance from the local Indians; in one particular tribe the women warriors were so fierce that they drove their male warriors in front of them with spears. Thus the river was named after the famous women warriors of the Greek myths, the Amazons. This makes Francisco de Orellana one of the unluckiest explorers of that age, as normally the river would have been named after him. This voyage also started our almost mystical wonder of the greatest river and the largest area of rainforest in the world, something we still og feel today. The Amazon River discharges approximately 20% of all | fresh water carried to the oceans. The Amazon drainage basin is the j| world's largest, covering an area of 7,050,000 km2 , about the size of 3 Europe. The river is a product of the Amazon monsoon, which every summer brings huge rains. This also produces the spectacular expanse of rainforest, which supports one of the highest diversity and largest number of species of any area in the world. The Amazon rainforest is also important when it comes to the future of global warming, as it is a huge natural store of carbon. Up until recently it was thought that an established rainforest such as the Amazon had reached maturity and thus could not take up any more carbon dioxide. Experiments in the heart of the Amazon rainforest have shown this could be wrong and that the Amazon rainforest might be sucking up an additional 5 tonnes of atmospheric carbon dioxide per ha per year. This is because plants react favourably to increased carbon dioxide; because it is the raw material for photosynthesis, the more of it the better. So having more carbon dioxide in the atmosphere acts like a fertilizer, stimulating plant growth. Because of the size of the Amazon rainforest it seems that presently it is taking up a large percentage of our atmospheric carbon dioxide
With biospheric feedbacks
- Without biospheric feedbacks
1850 1900 1950 2000 Year
Mean temperature at 1.5m
With biospheric feedbacks -----Standard run
1950 2000 Year
32. Met office model of CO2 concentration and mean temperature over time pollution, about three-quarters of the world's car pollution. But things could change in the future.
Global climate models developed at the Met Office Hadley Centre suggest that global warming by 2050 could have increased the winter dry season in Amazonia. For the Amazon rainforest to survive it requires not only a large amount of rain during the wet season but a relatively short dry season so that it does not dry out. According to the Hadley Centre model, global warming could cause the global climate to shift towards a more El Nin~o-like state with a much longer South American dry season. Kim Stanley Robinson in his novel Forty Signs of Rain uses the term Hyperni~o to refer to a new climate state. Hence, the Amazon rainforest could no longer survive and would be replaced by savannah (dry grassland) which is found both to the east and south of the Amazon Basin today. This replacement would occur because the extended dry periods would lead to forest fires destroying large parts of the rainforest. This would also return the carbon stored in the rainforest back into the atmosphere, accelerating global warming. The savannah would then take over those burnt areas, as it is adapted to coping with the long dry season, but savannah has a much lower carbon storage og potential per square mile than rainforest. So the Amazon rainforest | at the moment might be helping to reduce the amount of pollution j| we put into the atmosphere, but ultimately it may cause global 3 warming to accelerate at an unprecedented and currently unpredicted rate (Figure 32). However, we must still view this result with caution. First, the Met Office Hadley Centre model is unique, as it is the first model to have not only a fully coupled atmosphere and ocean system, but also a vegetation system which is fully coupled to the climate, so that climate influences the vegetation and the vegetation influences the climate. This is an extremely important step forward in climate models as ecologists have known for a long time that different vegetation types modify the local environment. This is especially true of the Amazon rainforest, which recycles at least 50% of the precipitation, maintaining a warm moist environment. At present there are no other GCMs to compare the results with. So until this happens we cannot place too much confidence in one model, but it does clearly indicate where we have to concentrate our scientific effort in the future. Second, the Met Office Hadley Centre model is one of many GCMs that show the world moving to a more El Nino-like state; but not all the GCMs reviewed by the IPCC show this shift. As the shift towards a more El Ni~o-like state is the key control on the future of the rainforest, it is something we need to have confidence in. As discussed before, confidence in science moves forward as a consequence of the weight of evidence and at the moment there is not enough convincing evidence that the world will move into a more El Ni~o-like state, or Robinson's Hyperni~o. Third, about 80% of the release of additional carbon from the terrestrial biosphere into the atmosphere predicted in the Hadley Centre model comes from increased soil decomposition which is a poorly understood process on the global scale. So until more GCMs have coupled vegetation models we cannot have confidence in this one prediction. But, as they say, watch this space!
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