A collection of papers published today have examined the impact global warming may have on weather patterns, food production and population by the end of this century.
Since the late 1990s, many researchers and policy makers have held a 2°C global temperature increase relative to pre-industrial times as a benchmark limit for global warming, saying that keeping warming below this threshold increases the likelihood that catastrophic changes can be avoided.
But it appears we are unlikely to meet that target, say researchers, and an average global warming of 4°C by the end of this century is more likely.
Appearing today in the Philosophical Transactions of the Royal Society A, the papers paint a picture of what a four-degree warmer world might look like, including changes in agriculture and water supply, ecosystems, sea level rise and the displacement of populations.
"People are talking about two degrees but the chances of actually delivering on that are pretty slim," says Mark New of Oxford University , United Kingdom, one of the researchers who compiled the collection.
"If we had a kind of a Marshall Plan to transform every major economy to a non-carbon based economy over the next 15 years, it's doable. But that's not going to happen. A lot of work suggests that the most likely outcome is between three and four (°C increase) with it very likely to be more than four."
Four degrees would only be a global average. Air over land will warm more than over the oceans, and some places will warm more than others.
Dry areas are likely to get drier, according to a study of water supply done by New and others, which could have severe implications for agriculture.
A team led by Peter Thornton of the International Livestock Research Institute used models to project the effect of a 4°C temperature increase on crop production in sub-Saharan Africa.
"The rate of crop failure in southern Africa increases to nearly one in every two years," says New of the study. "You can't continue to rely on your existing crops or practices. There's going to have to be some kind of a transformation."
"Most of these countries have low capacity to adapt," he adds.
Timing is everything
The timing of the temperature increase will be critical, as global population increases - predicted to peak later this century - will increase demand for food. "If you've got the peak in world population at exactly the same time when global temperatures peak, then clearly you've got a squeeze from both sides - the demand side and the supply side," says New.
New's colleague Fai Fung and others compared the projected global water supply at 4°C warmer with that at 2°C for the 120 largest river basins throughout the world.
"There's already a lot of water stress, so what we're saying is that in the future under these scenarios, it will probably get worse," says Fung. In general, the team's projections suggest that in a world that is 2°C warmer, population increases are more important than climate change in creating water stress, but once temperatures increase by four degrees, temperature increases become more important.
Some places, such as the Ganges river basin, may get wetter, the team's models project, but it could be that the wet season gets wetter, rather than that the water is distributed evenly. "It may be more water at the wrong time," says Fung.
Adaptation is key
Sea level rise is another key consequence of a warmer world. Research by a team led by Robert Nicholls at the University of Southampton , United Kingdom, suggests that four degree warming would result in one-half to two meters of sea level rise, although the greatest increases are less likely.
"I can't say we wouldn't see a half-meter with a two-degree rise," says Nicholls. "But it tends to make the likelihood of the really big changes a lot lower."
"The key question is how do we adapt," says Nicholls. "If we don't adapt, we're talking about tens of millions of people being forced to move from their homes, and huge areas of land being lost to the sea. Some of those areas would be London, Amsterdam, Shanghai and Mumbai."
Nicholls' work suggests up to 2.4% of the world's populations - 187 million people - could be forcibly displaced over the course of the century.
Adapting to the change, that is, building protection for such cities against rising waters, could save these areas. On the other hand, it would be expensive. Nicholls' team predicts it could cost up to 0.02% of global domestic product.
Researchers emphasised the importance of adaptation for other predicted changes, too.
"Some of the impacts could be overcome if society takes adaptive action, but the difference between adapting at two degrees and at four degrees is very different," New says. "There needs to be research into technologies to assist adaptation just as much as we need research into technology for moving out of a carbon based transportation system."
Whether because of sea level rise, water availability or food security, people will be displaced. Anticipating this is a key adaptation strategy, says Francois Gemenne of the Institute for Sustainable Development and International Relations in Paris, France.
"A key point is the importance of fostering the right to leave and the right to choose where," he said. "The majority of people lack the resources to leave and the social networks to do so."
What are the chances?
A study by Richard Betts at the Met Office Hadley Centre in Devon, United Kingdom suggests that a 4°C rise relative to pre-industrial levels could happen by the 2070s, if emissions are high, if the temperature response to those emissions is high, and if the feedback cycles to those emissions are high.
"Everything I'm talking about is within what's thought to be scientifically credible," he says.
If we reach 4°C higher by the 2070s, it would put us on track for an almost 7°C increase by the end of the century, he says. The projections are too uncertain to say exactly what trajectory we are on, says Betts.
And, Betts notes, there's nothing magical about 2°C or 4°C.
"I don't particularly sign up to the fact that 2°C is some kind of threshold for catastrophic climate change," he says. "It's all about risk assessment. The greater the warming, the larger the impact. The faster the warming, the harder it is to adapt to any impact. The risk becomes greater the more you put into the system."