We’re on pace for 4°C of global warming. Here’s why that terrifies the World Bank.

November 19, 2012

Over the years at the U.N. climate talks, the goal has been to keep future global warming below 2°C. But as those talks have faltered, emissions have kept rising, and that 2°C goal is now looking increasingly out of reach. Lately, the conversation has shifted toward how to deal with 3°C of warming. Or 4°C. Or potentially more.


Drought in Yunnan Province, China.

And that topic has made a lot of people awfully nervous. Case in point: The World Bank just commissioned an analysis (pdf) by scientists at the Potsdam Institute looking at the consequences of a 4°C rise in global temperatures above pre-industrial levels by 2100. And the report appears to have unnerved many bank officials. "The latest predictions on climate change should shock us into action," wrote World Bank President Jim Yong Kim in an op-ed after the report was released Monday. 

So what exactly has got the World Bank so worried? Partly it's the prospect that a 4°C world could prove difficult—perhaps impossible—for many poorer countries to adapt to. Let's take a closer look at the report:

1) The world is currently on pace for around 3°C to 4°C of global warming by the end of the century. In recent years, a number of nations have promised to cut their carbon emissions. The United States and Europe are even on pace to meet their goals. But those modest efforts can only do so much, especially as emissions in China and India keep rising. Even if all current pledges get carried out, the report notes, "the world [is] on a trajectory for a global mean warming of well over 3°C." And current climate models still suggest a 20 percent chance of 4°C warming in this emissions scenario.

2) The direct consequences of a 4°C rise in global temperatures could be stark. Four degrees may not sound like much. But, the report points out, the world was only about 4°C to 7°C cooler, on average, during the last ice age, when large parts of Europe and the United States was covered by glaciers. Warming the planet up in the opposite direction could bring similarly drastic changes, such as three feet or more of sea-level rise by 2100, more severe heat waves, and regional extinction of coral reef ecosystems.

3) Climate change would likely hit poorer countries hardest. The World Bank focuses on poverty reduction, so its climate report spends most of its time looking at how developing countries could struggle in a warmer world. For instance, a growing number of studies suggest that agricultural production could take a big hit under 3°C or 4°C of warming. Countries like Bangladesh, Egypt, Vietnam, and parts of Africa would also see large tracts of farmland made unusable by rising seas. "It seems clear," the report concludes, "that climate change in a 4°C world could seriously undermine poverty alleviation in many regions."

4) Yet the effects of 4°C warming haven't been fully assessed — they could, potentially, be more drastic than expected. Perhaps the most notable bit of the World Bank report is its discussion of the limits of current climate forecasts. Many models, it notes, make predictions in a fairly linear fashion, expecting the impacts of 4°C of warming to be roughly twice as severe as those from 2°C of warming. But this could prove to be wrong. Different effects could combine together in unexpected ways:

For example, nonlinear temperature effects on crops are likely to be extremely relevant as the world warms to 2°C and above. However, most of our current crop models do not yet fully account for this effect, or for the potential increased ranges of variability (for example, extreme temperatures, new invading pests and diseases, abrupt shifts in critical climate factors that have large impacts on yields and/or quality of grains).

What's more, the report points out that there are large gaps in our understanding of what 4°C of warming might bring: "For instance," it notes, "there has not been a study published in the scientific literature on the full ecological, human, and economic consequences of a collapse of coral reef ecosystems."

5) Some countries might not be able to adapt to a 4°C world. At the moment, the World Bank helps many poorer countries build the necessary infrastructure to adapt to a warmer world. That includes dams and seawalls, crop research, freshwater management, and so forth. But, as a recent internal review found, most of these World Bank efforts are focused on relatively small increases in temperature. 

This new World Bank report is less sure how to prepare for a 4°C world. "[G]iven that uncertainty remains about the full nature and scale of impacts, there is also no certainty that adaptation to a 4°C world is possible." That's why, the report concludes, "The projected 4°C warming simply must not be allowed to occur — the heat must be turned down. Only early, cooperative, international actions can make that happen."

So what sorts of actions might that entail? The International Energy Agency recently offered its own set of ideas for curbing greenhouse-gas emissions and keeping future warming below 2°C. That included everything from boosting renewable energy to redesigning the world's transportation system. But so far, nations have only made small progress on most of these steps.

Further reading:

--The full World Bank report is here.

--The International Energy Agency recently found that the world is seriously lagging on its climate goals, apart from a recent boom in solar and wind power. But there are plenty of constructive suggestions in the report.

 --Experts have warned that adaptation will be necessary even if the world does manage to limit global warming to only 2°C. (Greenhouse gases have already heated the planet about 0.8°C.) For more on that, see this earlier post looking at how even a 2°C world will likely see significant sea-level rise, albeit less than in a 4°C world.

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Neil Irwin · November 19, 2012