Europe’s exceptionally hot summer of 2003 is likely to be the norm by the end of the century, according to new climate research.1 Current extreme temperatures are likely to be exceeded in most years across Asia, Africa, the Middle East and Central America.
The stark warning comes from Professor David Battisti at the University of Washington in Seattle and Rosamond Naylor at Stanford University who predicted temperatures for 2080 to 2100. They found a greater than 90% chance of growing-season temperatures in the tropics and subtropics exceeding all records by 2090. In temperate regions, record highs will become normal.
Their forecasts are based on a scenario published in the latest Intergovernmental Panel on Climate Change report where greenhouse gas emissions peak in 2050 and carbon dioxide levels reach 700 parts per million by 2100.
Professor Battisti and Dr Naylor point out that heat-driven stress is often overlooked in agriculture in favour of water stress, even though yields can fall by 3-16% for each degree of warming.
For instance, during the 2003 heatwave, when the average summer temperature in France was 3.6°C above the long-term mean, maize, wheat and fruit yields in France and Italy fell by more than 20% compared with the previous year. In 1972, the Soviet Union’s grain production was down 13% after temperatures in the Ukraine hit record highs of 2-4°C above the norm.
Unprecedented seasonal average temperatures in the tropics and subtropics will hit the three billion inhabitants especially hard because many economies depend on agriculture.
In temperate regions, rising temperatures may boost yields, at least initially, and balance losses elsewhere. But the researchers say "even mid-latitude crops will probably suffer at very high temperatures in the absence of adaptation. Global climate change thus presents widespread risks of food insecurity." They call for major investment in developing crop varieties that tolerate extreme heat and water stress.
Meanwhile, UK research suggests ‘bio-geoengineering’ could help mitigate summer surface air temperature increases, at least in northern temperate regions.2 Crops with a high albedo - the ability to reflect sunlight into space - could be chosen, say a team of scientists at the University of Bristol.
Varieties of the same crop can have different albedos, the researchers say, because of variations in leaf waxiness, hairiness, variegation and canopies. Andy Ridgwell and colleagues calculated the effect of a 20% increase in crop albedo worldwide, which they say might be possible without damaging crop yields.
This increased albedo led to 0.1°C in avoided global temperature increase, in a climate model with CO2 at double the current concentration. But Dr Ridgwell says the result "would be much the same at current CO2 levels".
At a regional scale the cooling effect was much greater, with more than 1°C of avoided summer temperature increase across a wide band of temperate North America and Eurasia.
Dr Ridgwell admits more information is needed before bio-geoengineering can become reality. "We’re somewhat extrapolating from what’s known about crop albedo. We plan to take one or more major crop species and see what is possible."
In future, farmers could be encouraged to plant high-albedo crops in return for carbon credits because the cooling effect is equivalent to avoided emissions. Dr Ridgwell suggests farmers could earn about €23 per hectare, but the scheme would not qualify for credits under current systems and would not mitigate other CO2-related impacts such as ocean acidification.