Reducing greenhouse gas emissions soon and fast enough to
prevent a global average temperature rise of more than 2 °C
above the pre-industrial level commonly regarded as the
threshold for 'dangerous climate change' was always going to be
tough. As we approach the end of the first decade of the
twenty-first century and emissions continue to increase, keeping
below 2 °C with any reasonable probability will be possible only
with urgent and stringent mitigation measures. In practice, this
will necessitate almost immediate emissions reductions by rich
nations, followed soon after by reductions from developing
nations.
As the challenge of keeping below 2 °C increases, the likelihood
of reaching higher temperatures becomes correspondingly larger.
At the worst end of the scale, with continued intensive fossil
fuel use, temperatures could rise 4 °C by the 2070s, or even as
early as 2060 if there are strong positive feedbacks in the
carbon cycle. The situation is bleaker still once political
inertia is considered. Moderate-emissions scenarios, including
those arising from weak climate agreements, still result in a
significant probability of exceeding 4 °C by the end of the
century or early in the next century. As nations delay on
agreeing a global climate treaty, it seems essential to explore
the terra quasi-incognita of a world in which the average
temperature is 4 °C above the pre-industrial level, and to
understand the implications for nature and society.
Brave new world
A world where the average temperature was 4 °C higher than in
pre-industrial times would be very different from the one we now
inhabit, and even from one with 2 °C of warming. Studies suggest
that 2-4 °C of warming would trigger the permanent break-up of
the Greenland ice sheet, causing sea level to rise by up to
seven metres in the long term. With warming of 3 °C, the Arctic
Ocean would most likely be ice-free in summer. At 4 °C, most
reef-building corals would be unable to adapt to changes in
ocean temperature and acidification, in which case tropical
coral reefs would die out or become far less diverse. While
thresholds or tipping points in other systems are less well
known, the risk of major shifts in ecosystems such as tropical
forests increases as global temperature rises from 2 to 4 °C.
A 4 °C, the world would probably be warmer than any time in the
last 800,000 years10 and certainly the last 18,000 years, the
period in which modern humans evolved. Moreover, the rate of
climate change would be as fast as or faster than any previously
experienced. Because land areas warm faster than the ocean and
higher latitudes more than lower latitudes, temperature
increases would exceed 4 °C in many regions. Approximately 13
per cent of land including the Amazon, the Sahara-Sahel-Arabia
region, India and northern Australia could experience average
temperatures for which there are no spatial analogues in today's
climate; in other words, the temperature in these regions would
be higher than the average at any place on Earth today.
Correspondingly, present-day climates in the tropics and
subtropics would shift short distances to higher elevations or
in some cases several thousands of kilometres polewards.