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