Forestclimate interactions and feedbacks

Various interactions and feedbacks are in play between forests and climate. Extensive changes in the area of forests due to deforestation can seriously affect the climate in the region of change. Changes in carbon dioxide, temperature or rainfall associated with climate change can have a major impact on the health or structure of forests that can in turn feed back on the climate. We consider some of these effects in turn.

Changes in land use such as those brought about by deforestation can affect the amount of rainfall, for three main reasons. Over a forest there is a lot more evaporation of water (through the leaves of the trees) than there is over grassland or bare soil, hence the air will contain more water vapour. Also, a forest reflects 12-15% of the sunlight that falls on it, whereas grassland will reflect about 20% and desert sand up to 40%. A third reason arises from the roughness of the surface so stimulating convection and other dynamic activity where vegetation is present.

It was in fact an American meteorologist, Professor Jules Charney, who suggested in 1975 that, in the context of the drought in the Sahel, there could be an important link between changes of vegetation and rainfall. Early experiments with numerical models that included these physical processes demonstrated the effect and indicated large reductions of rainfall when large areas of forest were replaced by grassland. In the most extreme cases, the rainfall reduction was so large that grassland would no longer be supported and the land would become semi-arid.

However, even in the absence of changes of vegetation because of human action, interactions occur between the climate and the forest that can effect large changes. Three important feedbacks that lead to reduced precipitation are:

• increased carbon dioxide causes stomatal closure within the leaves of the trees so reducing evaporation;

• increased temperature tends to cause forest dieback, again leading to reduced evaporation;

• increased temperature causes increased respiration of carbon dioxide from the soil so leading to further global warming - the climate/carbon-cycle feedback mentioned in Chapter 3.

For Amazonia, when these three feedbacks are added to the effect on the forest of the local climate change that is likely to occur because of global circulation changes, under a scenario of continuing increase in carbon dioxide emissions, simulations suggest major losses of forest cover in the Amazon basin during the twenty-first century. Large areas would be replaced by shrubs and grasses and part of Amazonia could become semi-desert.49 Such results are still subject to considerable uncertainties (for instance, those associated with the model simulations of El Niño events under climate change conditions and the connections between these events and the climate over Amazonia), but they illustrate the type of impacts that might occur and emphasise the importance of understanding the interactions between climate and vegetation.

Arolla pine

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Norway spruce

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Figure 7.16 Simulated environmental realised niches (the realised niche describes the conditions under which the species is actually found) for three tree species, Arolla Pine, Norway Spruce and Common Beech. Plots are of biomass generated per year against annual means of temperature and precipitation. Arolla Pine is a species with a particularly narrow niche. The narrower the niche, the greater the potential sensitivity to climate change.

The above discussion has related to the impact of climate change on natural forests where the likely impacts are largely negative. Studies of the impacts on managed forests are more positive.51 They suggest that with appropriate adaptation and land and product management, even without forestry projects that increase the capture and storage of carbon (see Chapter 10), a small amount of

Current Arctic conditions

Projected Arctic conditions

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Boreal forest

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Figure 7.17 I ce cover and vegetation in the Arctic and neighbouring regions as observed today and as modelled for 2080-2100 under the IS 92a scenario. Latest projections are for complete disappearance of Arctic sea-ice in late summer, possibly by 2020.

climate change could increase global timber supply and enhance existing market trends towards rising market share in developing countries.

Large changes are also expected in the polar regions (Figure 7.17) both in the amount of sea-ice cover and in vegetation with large implications for managed and unmanaged ecosystems both on land and marine areas. Changes in drylands, in their extent or the nature of their vegetation are also of great concern (see box on page 197).

A further concern about natural ecosystems relates to the diversity of species that they contain and the loss of species and hence of biodiversity due to the impact of climate change. Significant disruptions of ecosystems from disturbances such as fire, drought, pest infestation, invasion of species, storms and coral bleaching events are expected to increase. The stresses caused by climate change, added to other stresses on ecological systems (e.g. land conversion, land degradation, deforestation, harvesting and pollution) threaten substantial damage to or complete loss of some unique ecosystems, and the extinction of some endangered species. Coral reefs and atolls, mangroves, boreal and tropical forests, polar and alpine ecosystems, prairie wetlands and remnant native grasslands are examples of systems threatened by climate change. In some cases the threatened ecosystems are those that could mitigate against some climate change impacts (e.g. coastal systems that buffer the impact of storms). Possible adaptation methods to reduce the loss of biodiversity include the establishment of refuges, parks and reserves with corridors to allow migration of species, and the use of captive breeding and translocation of species.52

So far we have been considering ecosystems on land. What about those in the oceans; how will they be affected by climate change? Although we know much less about ocean ecosystems, there is considerable evidence that biological activity in the oceans has varied during the cycle of ice ages. Chapter 3 noted (see box on page 43) the likelihood that it was these variations in marine biological activity which provided the main control on atmospheric carbon dioxide concentrations during the past million years (see Figure 4.6). Changes in ocean water temperature and possible changes in some of the patterns of ocean circulation will result in changes in the regions where upwelling occurs and where fish congregate. Some fisheries could collapse and others expand. At the moment the fishing industry is not well adapted to address major change.53

Some of the most important marine ecosystems are found within coral reefs that occur in many locations throughout the tropical and sub-tropical world. They are especially rich in biodiversity and are particularly threatened by global warming. Within them the species diversity contains more phyla than rainforests and they harbour more than 25% of all known marine fish.54 They represent a significant source of food for many coastal communities. Corals are particularly sensitive to sea surface temperature and even 1 °C of persistent warming can cause bleaching (paling in colour) and extensive mortality accompanies persistent temperature anomalies of 3 °C or more. Much recent bleaching, for instance that in 1998, has been associated with El Niño events.55

Added to the stresses caused by climate change will be those that arise from increased ocean acidification that results from the increase of carbon dioxide in ocean water (Figure 7.18). These increased stresses will be most serious for a wide range of planktonic and shallow benthic marine organisms that use aragonite to make their shells or skeletons, such as corals and marine snails (pteropods). Research on how serious these stresses will be is being actively pursued.56

Coral bleaching is a vivid sign of corals responding to stress which can be induced by increased or reduced water temperatures, increased solar irradiance, changes in water chemistry, starvation caused by a decline in zooplankton levels as a result of over-fishing, and increased sedimentation.

Summarising the impacts of climate change on ecosystems with a warming of global average temperature of 2 °C or more from its pre-industrial value, there are five areas of particular concern.57

(1) The resilience of many ecosystems (their ability to adapt) is likely to be exceeded by an unprecedented combination of change in climate, associated disturbances (e.g. flooding, drought, wildfire, insects, ocean acidification) and in other drivers such as land-use change, pollution and over exploitation of resources.

(2) The terrestrial biosphere is currently a net carbon sink (see Table 3.1). As was mentioned in Chapter 3, during the twenty-first century, it is likely to become a net carbon source thus amplifying climate change.

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Figure 7.18 Rising CO2 concentration in the atmosphere leads to move CO2 dissolved in the ocean with rapid increase in ocean acidity (lower pH) to levels not encountered for millions of years. Past (blue spots, data from Pearson and Palmer 2000) and contemporary variability of marine pH (purple spots, with dates). Future predictions are model derived values assuming atmospheric carbon dioxide concentration of 500 ppm in 2050 and 700 ppm in 2100.

(3) Approximately 20-30% of plant and animal species so far assessed (in an unbiased sample) are likely to be at increasingly high risk of extinction.

(4) Substantial changes in structure and functioning of terrestrial ecosystems are very likely to occur with some positive impacts due to the carbon dioxide fertilisation effect but with extensive forest and woodland decline in mid to high latitudes and the tropics associated particularly with changing disturbance regimes (e.g. through wildfire and insects).

(5) Substantial changes in structure and functioning of marine and other aquatic ecosystems are very likely to occur. In particular the combination of climate change and ocean acidification will have a severe impact on corals.

Human health is dependent on a good environment. Many of the factors that lead to a deteriorated environment also lead to poor health. Pollution of the atmosphere, polluted or inadequate water supplies and poor soil (leading to poor crops and inadequate nutrition) all present dangers to human health and well-being and assist the spread of disease. As has been seen in considering the impacts of global warming, many of these factors will be exacerbated through the climate change occurring in the warmer world. The greater likelihood of extremes of climate, such as droughts and floods, will also bring greater risks to

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