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Home truths about global warming

Tip O'Neill, speaker of the US House of Representatives for the decade to 1987, famously declared, "All politics is local." Dr Richard Betts, ecosystems and climate impacts manager of the UK Meteorological Office's Hadley Centre for Climate Prediction and Research, adds that so too is the weather.

"We care about climate change where we are, not globally," Betts stated in April at the Royal Society scientific discussion meeting Food Crops in a Changing Climate. Yet global and regional climate models address global warming with a strong emphasis on such global drivers as greenhouse gases and aerosols. "We generally model only those processes that are important for large-scale climate," he added.

Left out of the models are processes with local effects, which may aggregate to influence the global picture. Most of the local drivers have to do with land use. "Scenarios of greenhouse gas emissions implicitly assume changes in land use," Betts pointed out, "but the direct effects of these changes are not yet routinely considered in climate change projections."

Local drivers

Urbanisation creates warm microclimates
Urbanisation creates warm microclimates

Two of the broadest "local" climate change drivers are the ongoing and anticipated replacement of forest cover with cropland in Amazonia, which could reduce rainfall in the lower reaches of the Amazon River by a millimetre a day, and expansion of the area under crops in China and continental Southeast Asia. Urbanisation is well known to generate a warm microclimate, but changes in the type of vegetation that covers an area can also have strong effects. Different dominant plant species differ in their surface albedo, or the percentage of sunlight that is reflected back up into the atmosphere or beyond into space. Latent heat and moisture rises into the atmosphere from different kinds of vegetation at different rates, and the density of cover affects how much soil moisture is available to evaporation. The aerodynamic roughness of vegetation-atmosphere interface affects the circulation of air and so the rate at which heat and moisture are transferred.

Confusingly, crop irrigation has opposite effects on different scales. It increases water vapour, the most important greenhouse gas, and so contributes somewhat to radiative forcing of global warming. At the same time, irrigation brings cooler surface temperatures locally. The world's most widely irrigated crop is rice, and the role of feedback effects from the greenhouse gas methane, which rises from rice paddies and other croplands, is an open question. The greenhouse gas carbon dioxide is both captured and emitted by crops, which therefore influence where much of the world's carbon is stored.

Carbon-cycle feedbacks

Observing that soil and vegetation carbon drop as atmospheric CO2 rises, Betts wonders what role croplands play in the carbon-cycle feedbacks that accelerate global warming. "Specifically," he asked, "how do farmers affect carbon cycle feedbacks with their choice of cultivar, irrigation systems, tillage practices and other factors we haven't yet thought to consider?"

CO2 is the Peter Sellers of climate change because it plays so many roles, often at crossed purposes when viewed from the farmers' perspective. Its starring role in radiative-driven climate-change models is to intensify the greenhouse effect by trapping heat that would otherwise radiate into space. A counter effect of heightened CO2 concentrations is to fertilise plant growth, though apparently less than hoped. The local effects of the gas are equally complex. By inducing plants to close their stomata, the pores by which gases and moisture enter and leave foliage, CO2 seems to protect crops from local concentrations of poisonous ozone. The same effect on stomata has a direct impact on the water cycle by making plants retain moisture. As a result, they take up less water, which increases runoff and the risk of flooding.

Sea level rise

Crops and their management may even affect sea level rise. Potentially influencing sea levels as much as the combined effects of seawater thermal expansion and the melting of glaciers and icecaps are other forms of terrestrial storage. How much water is held back from the sea is directly affected by groundwater mining and runoff impoundment in reservoirs, both of which mainly service irrigation systems. Deforestation to clear land for agriculture and other factors affecting evaporation rates also affect the global water budget. Yet they are typically ignored by existing climate change models.

"We don't even know if the missing variables would make the prognosis better or worse," Betts confessed. "What we need is a fully coupled climate-carbon-chemistry-hydrology-crop earth system model." The computational challenges of such a system are vast, but the Hadley Centre has begun to include many local drivers in its global climate model.

"Models need to simulate local as well as global drivers of climate change," Betts urged. "These can have important local impacts and should be counted even if they have little or no large-scale effect."

Date published: July 2005


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