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What is a 4°C world?

• 22 Nov 2012, 14:30
• Freya Roberts

This week the World Bank warned that without action, nations are on track for a 4°C world. But what does that mean? We take a look at how climate models predict a four degree world, and what it might look like.

The concept of a four degree world is essentially a tool, used to illustrate what could happen if countries don't step up current ambitions to cut greenhouse gas emissions. And it's becoming increasingly plausible.

New reports highlight that emissions are higher than ever, current pledges to mitigate climate change aren't doing enough, and the gap between what's needed and what's being done continues to widen.

Predicting a four degree world

The idea of a four degree world refers to what climate models predict the world could look like by 2100 when, averaged over all of earth's surfaces, temperatures rise by four degrees Celsius compared to pre-industrial levels.

To look into the impacts of a four degree temperature rise, scientists must first make assumptions about how this could happen. That means figuring out how fast greenhouse gas emissions will increase in the future.

The most recent report from the Intergovernmental Panel on Climate Change uses a number of emissions scenarios to give a range of plausible temperature outcomes. The worst-case scenario, the A1FI scenario, projects temperatures are likely (with a 66% chance) to rise somewhere between 2.4 and 6.4 degrees Celsius by 2100. The best estimate within this range is four degrees.

Global average surface temperature rise under climate change scenarios relative to the 1980-1999 average. Source: IPCC Fourth Assessment Report SPM

Newer studies use different types of scenarios, called Representative Concentration Pathways (RCPs), to predict future climate change. Climate models using the highest of these new concentration pathways, called RCP8.5, suggests four degrees warming as a best estimate for a world scenario where little or no mitigation action is taken.

Global average surface temperature rise relative to the 1986-2005 average under new Representative Concentration Pathways. Source: Knutti & Sedláček, 2012

Although other scenarios can be used, research exploring the impacts of a four degree world are largely based on climate model projections from these two high emissions scenarios - the A1F1 scenario and the RCP8.5.

What would a four degree world look like?

Most of the world's human population would experience more than the global average in a four degree world, and that's largely down to the fact that land warms more quickly than ocean. Since four degrees is a global average surface temperature, it includes the surface of both land and water.

The latest climate model projections using RCP8.5 show that most of the world's large land masses will experience somewhere between four and seven degrees Celsius of warming. In many places, but not all, it appears warming would be greater in winter months.

For the northern hemisphere, that means stronger warming during December, January and February (DJF in image below) and for the southern hemisphere that means stronger warming during June, July and August (JJA in image below)

Global distribution of warming under a four degrees temperature rise, as projected by RCP8.5. Source: Knutti & Sedláček, 2012

The polar regions and high latitudes, where many of the biggest expanses of land ice exist, are also expected to warm faster than the oceans. Both polar regions are expected to warm more during their winter months, but changes in the Arctic are especially pronounced.

The Arctic sea and surrounding land masses are predicted to become between seven and 11 degrees warmer - this is the most extreme temperature rise in the global picture. Almost at the other end of the scale, the model predicts the UK will warm by two and three degrees during winter months, and around the global average of four degrees during summer months.

It's worth remembering these conclusions are drawn from one study looking at changes on a pretty broad scale. Predictions of temperature rise using these new concentration pathways are just starting to emerge, and future research using regional climate models will be able to scale down these results more accurately.

But global models like this help explain what a four degree world might actually look like, and highlights the differences between the oceans, continents and polar regions.

Why are we talking about a four degree world?

Despite international pledges to limit global temperature rise to two degrees, a new report from the World Bank calculates that the measures suggested to tackle climate change are unlikely to keep temperature rise below three degrees. The report concludes:

"Even with the current mitigation pledges fully implemented, there is roughly a 20% likelihood of exceeding 4°C by 2100. If they are not met, warming of 4°C could occur as early as the 2060s"

Added to this, a report released yesterday by the United Nations highlights that the emissions gap - between what's needed and what's actually being done to mitigate climate change - is continuing to grow. This means lower estimates of temperature rise are becoming increasingly unlikely.

Since 2009, scientists have looked more closely at what a climate changed world with little or no mitigation might look like. The Met Office in 2011 produced an interactive map showing potential changes in extreme temperatures and precipitations, highlighting the effects that could have on humans, crops and marine life.

This map, and many other studies, identify sea level rise as one of the most certain responses to four degrees warming. Globally averaged, that much warming could translate to sea level rise of between 0.5 and two metres, depending on the rate of melting from ice on land. The middle estimates from the new RCP8.5 puts sea level rise at 1.1 metres by 2100.

Many of the the other impacts are harder to accurately predict, especially at higher levels of warming, because it's not certain how feedbacks in the climate system could respond. But these impacts are likely to be much more severe than they would be in the two degree world that many governments pledged.

The new World Bank report concludes that a four degree world would be one of "unprecedented heatwaves, severe drought and major floods in many regions". The risks of doing nothing are clearly very high, and increasingly plausible without stronger efforts to reduce carbon emissions.