From shrinking glaciers to sea water becoming more acidic, climate change has already a notable imprint on Earth’s natural systems, according to a United Nations climate-science report released today. The warming projected for the 21st century poses risks — which will be severe if the rise in atmospheric concentration of heat-trapping gases continues unabated — to future food security, human well-being and wealth, says the Intergovernmental Panel on Climate Change (IPCC).
The document, which was the work of 243 lead authors, 66 editors and 436 additional experts, is the first IPCC report in seven years on the impacts of global climate change.
More frequent heat spells and drought, and more erratic precipitation, threaten to reduce freshwater water supplies and crop yields on all populated continents and particularly in countries in dry regions and the tropics. On land and in the oceans, biodiversity will decline as plants and corals die and species unable to adapt migrate or become extinct, the report concludes.
“No one on this planet will be untouched,” said chairman Rajendra Pachauri, opening the press conference that followed a week-long meeting to finalize the report.
Chris Field, a grassland ecologist at the Carnegie Institution for Science at Stanford University in California, and co-chair of the working group that drafted the document, says that the key to the report is “how many impacts have been quantified. These are no longer projections — these are impacts,” he says.
“It’s changed the debate from one about the future to one about the present,” says Mark Howden, an agriculture expert at the Commonwealth Scientific and Industrial Research Organisation in Canberra, who was lead author on the report’s chapter on food security.
The data have helped the author–scientists to paint a picture of unprecedented clarity of some impacts of climate change. Warming linked to human activity, for example, is prompting species of marine life to migrate to higher latitudes, disturbing food chains and habitats there. This has consequences on fishery productivity in already over-fished parts of the oceans, the report warns. In tropical seas, species migration is projected to reduce the catch of economically important fish by more than 50%.
Food-crop studies also point to potentially significant drops in yield in maize (corn) and wheat as climate change proceeds, especially in tropical areas. Soy and rice — thought to be buffered from temperature extremes by the water in the paddies — are thought to be less susceptible to rising temperatures.
The report mapped spikes in grain prices resulting from climate extremes in recent years — something that Howden says shows the impact as a trend even if a causal link can’t be established in any single case.
The problem could be especially grave for farmers in the more vulnerable areas in the lower latitudes, he says. Although climate change could force a gradual restructuring of land use — for example, driving some farmers to shift to raising livestock — a more serious problem will arise when those vulnerable populations are hit by increasingly wild climate fluctuations and more frequent extremes. “Really big droughts will trash production,” says Howden. For poorer countries, especially those without proper storage infrastructure, that could spell disaster, he says.
But the data are still not easily interpreted. For example, 10% of the projections the IPCC has considered predict a 10–50% decline in global crop yields between 2030 and 2049, compared to yields in the late 20th century. Another 10% of the studies predict a decrease in crop yields of more than 25% (up to roughly 50%) during the same period. However, almost 20% of projections project yield gains of up to 25% for the same period.
The seeming discrepancy, says Hermann Lotze-Campe, an agricultural economist at the Potsdam Institute of Climate Research in Germany, mirrors the complexity of the processes involved in photosynthesis and plant growth. Results from lab experiments and controlled free-air carbon-enrichment experiments, provide important insight but cannot simply be extrapolated to real-world changes in agriculture, as those are influenced by many other factors. Furthermore, he says, the fertilization effect of elevated carbon dioxide concentrations on plant growth is not sufficiently well understood.
The report also brought a new level of detail to studies of the oceans. Marine ecosystems had been neglected in the past, says Field, because sea-water temperatures change more slowly than atmospheric ones and animals have more freedom to move there than they do on land. Thanks to a flood of data on the subject, ocean acidification, essentially mentioned only in passing in the last report, in 2007, is now a central theme, says Jean-Pierre Gattuso, an oceanographer at the National Centre for Scientific Research in Villefranche-sur-Mer, France.
Ocean acidification, already apparent in large patches of the global ocean, will combine with warming and other changes, putting at risk dozens of species of warm- and cold-water corals, mollusks and crustaceans. “Meta-analyses and expert surveys have demonstrated that we know enough about the future combined impacts of ocean acidification and warming that it is critical to act now to reduce these impacts,” says Gattuso.
The report has been steeped in controversy, with some scientists labelling it alarmist and others saying that it underestimates the impact of climate change. But it has tried in many ways to strike a compromise, pointing to the need to mitigate some causes of climate change while at the same time recognizing the benefits that can come from various strategies of adapting to it. With 12,000 citations in its 30 chapters, the authors have staked a claim that it covers a suitably broad spectrum of scientific findings. “I hope the this report will reveal what we know and what we don’t know and that it will start a conversation,” says Field.