Global Warming FAQ - What are the Predictions for the Future?

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Global Warming
Answers to Frequently Asked Questions

What are the Predictions for the Future?	Return to main FAQ
Are climate models accurate? Will increased plant growth absorb the excess CO₂?

Are climate models accurate?	TOP
Early climate models were very crude, and were only able to demonstrate the basic principles – that the greenhouse effect would raise temperatures. Never the less, even by 1998 they were able to simulate the main features of our climate, including, according to a pamphlet from the World Meteorological Association, 'the seasonal cycle of temperature, the formation and decay of the major monsoons, the seasonal shift of the major rain belts and storm tracks, the average daily temperature cycle, and the variations in outgoing radiation at high elevations in the atmosphere as measured by satellites. Similarly, many of the large-scale features observed in the ocean circulation have been reproduced by climate models'. Modern models, such as the most recent update to HADCM3, are able to recreate the climate record without the need for artificial forcings (termed 'flux adjustments). The following are some recent examples of climate model successes: The predictions from climate models of general changes in global precipitation are apparently being borne out (see above). Models predict changes in water availability in the US - mainly because warmer winters will raise the snowline. According to this report, observations back up the models. An increasing percentage of California's precipitation over recent decades is falling as rain rather than snow and, in the last 50 years, run-off peaks in the western US and Canada have been happening earlier and earlier. The cause seems to be a region-wide trend towards warmer winters and springs. In the UK, climate models predict that rainfall will become more intense (that is, heavy rainfall will occur more often) and will become greater in winter months. Recent analyses of rainfall patterns over the past 50 years have confirmed that UK rainfall is changing in this way. The shifting of the North Atlantic Oscillation to a predominantly positive phase (see above) is "consistent with most climate models' response to greenhouse gases" according to a report at the American Geophysical Union meeting in December 2001. Two recent studies, published in the journal Science, have successfully modelled the observed changes in ocean temperature. Similarly, models predicted the loss of dissolved oxygen in the Antarctic ocean. A controversy regarding the ability of climate models to simulate ancient warm periods that the earth has been resolved in favour of the climate models. Ocean sediments seemed to record a greenhouse climate with cool tropical oceans; model results implied that the ocean surfaces were rather warmer. However, recent studies have shown that the climate models were right all along - and that it was our understanding of past climate that was at fault. Nevertheless, there is still a great deal of uncertainty in predicting future climate. Modern models are still very far from perfect, and they still contain a large number of uncertainties. Examples of areas in which models still fail to fully explain the modern climate data include the relative lack of warming of tropospheric temperatures, the intense warming of the Antarctic Peninsula, changes in tropical cloud cover, and fine scale fluctuations in temperature. Although there is plenty of room for error, it is important to realise that the effect of any errors is unknown. That is, it could be that warming is less severe than predicted. However, and just as likely, the warming could be more severe than predicted. A good article on the problems with models is available from the Geophysical Fluid Dynamics modelling at Princeton (although it is a little out of date since it was written in 1998). According to the IPCC report, current estimates for the next 100 years range from 1.4–5.8°C, depending on the model used and the carbon production and carbon cycle model used. The most recent estimate of future global warming, produced by amalgamating the output from a number of different models, predicts that in the absence of climate-mitigation policies, there is a 90% chance that the earth will warm by 1.7° to 4.9°C between 1990 and 2100 (Wigley & Raper, 2000 News Report, Paper).
Will increased plant growth absorb the excess CO₂?	TOP
Satellite observations suggest that plant growth has been increasing, although thought that this is mainly due to rising temperatures and increased rainfall, rather than a direct effect of increased CO₂ (see this article from the Earth Observatory). Whatever the cause, the resultant increase in plant mass captures some CO₂ from the atmosphere. The effect of this can be seen in assessments in carbon balance – the natural world is already absorbing more than it emits, and predictions of climate change are based on the assumption that this effect will continue. Unfortunately, there are several problems. Firstly, it's not enough. Data from Mauna Loa show that carbon dioxide levels have steadily increased throughout the century. Studies have shown that the ability of trees to soak up excess carbon is limited by the availability of other nutrients, as discussed in this article from National Geographic. Secondly, there's more to greenhouse gas than CO2, as this link shows. Methane, a very important greenhouse gas, has doubled in concentration since the start of the industrial age. Thirdly, we are actively reducing biomass by cutting down forests and draining marshland – deforestation in the 1980s was responsible for around 1.6 to 1.8 GtC emissions, according a report from the Edinburgh Centre for Carbon Management. In the 1990s, changing land use meant that deforestation has been balanced by reforestation of disused farmland, and there has been a net absorption of 1.4 billion tonnes. However, as these new forests mature, it is predicted that the balance will shift once again to a net loss of CO₂ to the atmosphere. Fourthly, although forests contain more CO₂ than grassland, they are darker. This means that they absorb more heat. Scientists from the USA's Lawrence Livermore Laboratories report that deforestation likely played an important part in the global cooling that took part over the last millennium. Finally, and most importantly, climate change is likely to have important effects on the carbon cycle. For example, as the oceans warm they will release CO₂, and as the permafrost thaws it will start to decay, releasing large amounts of CO₂. Peat marshes, too, are likely to release increasing amounts of CO₂ as they warm (see this news report). Although the exact effects of these changes are hard to calculate, the current estimate from the HADCM3 model is that the net effect will be to increase global by a further 3ºC over the next 100 years – making the total increase up to 8ºC. (A similar effect is thought likely to explain the ends of ice ages, as discussed above).

Other Global Warming FAQ Topics

Return to main FAQ

Is the Earth getting warmer?
Have surface temperatures risen?
Is the observed temperature rise due to urban heat islands?
Is the observed temperature rise a artefact of changes in coverage?
Do satellite data show that the earth is not warming?
Are the mountain glaciers melting?
Is the Antarctic warming?
Is the Arctic warming?
Is Arctic ice melting?
Is the permafrost thawing?

Are the oceans warming?
Are the corals dying?
Is the sea level rising?
Is the rise in sea level normal?
Is the North Atlantic (Arctic) Oscillation behaving normally?
Are precipitation patterns changing?

How does the current climate compare with that of the past
How does the current temperature compare with the past 1000 years?
How has temperature and CO₂ changed since the last ice age?
How does the current temperature compare with the past 400,000 years?
How does the current temperature compare with the past 600,000,000 years?
Have rapid increases in CO₂ caused climate change in the past?

What is causing the increased warmth?
Is there a natural greenhouse effect?
Is water vapour the most important greenhouse gas?
Are greenhouse gases increasing?
What is causing the increase in CO₂ and other greenhouse gases?
Is the Earth absorbing more radiation than it emits?
Is the recent warming caused by changes in solar activity?
Is the recent warming caused by changes in volcanic activity?
What caused the global temperature changes of the 20th century?

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Last updated 29/07/05. By Tom Rees. Contact the author