A comparison of climate impacts across the world
by the end of the century under RCP2.6 and RCP8.5. (a) Average
surface temperature, (b) average annual rainfall, (c) Arctic sea
ice extent and (d) change in ocean pH. Source:

Two degrees is the internationally-agreed target
for limiting global warming, and has a
long history in climate policy
. Ambition that we can still
achieve it is running high as climate negotiators gather in Lima to
lay the groundwork for a potential global deal in 2015.

But against this optimistic backdrop, greenhouse
gas emissions have continued to rise. With each passing year the
scale of the task looms ever larger. There are very
real questions about
whether or not the world will be able to stay below the two degree

So what happens if we fail to meet the two degree target? What
would it mean to resign ourselves to a post-two degree world? And
if not two degrees, then what?

As temperatures rise, so do the

Two degrees
above pre-industrial temperature
has been agreed by countries as an
appropriate threshold
which climate change risks become unacceptably high.

Global temperature has risen 0.85 degrees
Celsius since 1880, according to the latest
from the Intergovernmental Panel on Climate
Change (IPCC).

We could be due another couple of tenths on top of
that as past emissions take a
decade or so
to reach their full effect warming.
Together with current and expected emissions we’re essentially
already committed to about one degree of warming, scientists


Observed global mean temperature from 1850 to
2012, relative to the 1961-1990 average. Coloured lines represent
three different datasets. Top panel shows yearly averages, bottom
shows decadal averages. Source:
IPCC 5th Assessment Report

While the international community uses two
degrees as the rule-of-thumb threshold for “dangerous” warming,
some climate impacts are already locked-in, particularly for
low-lying and island nations. Professor Anders
Levermann from the Potsdam Institute for Climate Change Research
tells us:

“At one degree we are
already experiencing damages. Sea level rise in the long term …
is somewhere in the vicinity of two metres. That puts cities
like New York, Calcutta and Shanghai in difficult positions, and
they need to protect themselves.”

Rising temperatures have
consequences for food and water security, infrastructure,
ecosystems, health and the risk of conflict,
says the IPCC
. And the higher
the temperature, the greater the risk those climate
change impacts will be serious and damaging.

One of the most direct impacts society feels
from climate change is the greater frequency and intensity
extreme weather
.In Europe,
heatwaves like the 2003 event which killed
70,000 people
are already ten times more
than a decade ago. In the UK, climate
change is making extreme wet winters like last year’s about

25 per cent more likely
, scientists

Given that impacts scale with rising
temperature, two degrees of warming above pre-industrial levels is
thought to represent a climate target that’s both achievable and
doesn’t expose us risks that are too difficult to

But suppose we collectively decide the task of
keeping to this target is too great, or the price of swift
mitigation too high. What are the consequences of
exceeding our self-imposed limit?

Three degrees

The IPCC uses four pathways to
illustrate how greenhouse gases could evolve this century. The
lowest, RCP2.6, is designed to show how warming could be kept below
two degrees above pre-industrial levels (blue line in the graph

(Note that temperatures in the graph are relative to the 1986 to
2005 baseline, so add 0.61 degrees to get warming above
pre-industrial levels.)

If we aim instead to stay in line with IPCC’s
second scenario, RCP4.5, that should see global temperature level
out at about three degrees above pre-industrial levels (green

Extended RCPs

Extension of the IPCC’s emissions scenarios to
2300. RCP2.6 limits warming to two degrees above pre-industrial
levels (blue), RCP4.5 levels out as about three degrees (green). In
RCP8.5, temperatures exceed four degrees by 2100 and continue to
rise. Meehl et al., (2013)

But even accepting this level of risk would
require a strong commitment to mitigation. In the IPCC’s three
degrees scenario, global emissions peak around 2040 and start to
decline. But the risks at three degrees are already very high, says

“Three degrees of
warming increases the risk of strong sea level rise from, for
example Antarctica, or the collapse of marine ecosystems, such as
Arctic sea ice or coral reefs … [It] increases the risk of
intensification of extreme events … In short, beyond two degrees
of warming we are leaving the world as we know it.”

Four degrees

In the IPCC’s most extreme scenario, RCP8.5,
global temperature reaches more than four degrees above
pre-industrial levels by 2100. And unless emissions cease
altogether, temperatures will continue to rise long past the end of
the century.

With emissions accelerating faster than they are
now for the next few decades, global temperature rise in RCP8.5
reaches five degrees by about 2120 and six degrees by 2150. This is
a worst-case scenario, says Levermann, but that doesn’t mean it’s
not a possibility.

Professor Richard Betts from the UK’s Met Office
is the coordinator of a new international project called Helix,
which looks at the impact of very high levels of warming. He tells

“[I]t’s very difficult
indeed to know what a two degree world will look like, let alone
four degrees or even six.”

But as the latest IPCC report notes, it’s clear
the risk of triggering very large, abrupt or irreversible changes
in the climate system increases the higher temperatures

“With increasing
warming, some physical systems or ecosystems may be at risk of
abrupt and irreversible changes … Risks increase disproportionately
as temperature increases between one to two degrees Celsius of
additional warming and become high above three degrees

Since temperatures have risen almost one degree
already, three degrees “additional warming” here means about four
degrees above pre-industrial levels in total.

Continued warming will at some stage trigger the
Greenland ice sheet to gradually collapse, although scientists
can’t say precisely at what temperature this would occur, the IPCC
report says:

“For sustained warming
greater than some threshold, near-complete loss of the Greenland
ice sheet would occur over a millennium or more, contributing up to
seven metres of global mean sea level rise.”

Natural ecosystems are also set to suffer under
higher temperatures, Betts tells us:

“Most life
on Earth tends to be adapted to its current conditions, so if
conditions change then species either need to move … or adapt to
new conditions, or die out. [The] resilience of the natural world
seems to be being reduced as a direct consequence of other human
actions, through land use and habitat loss, so it’s a double-whammy
for ecosystems.”

Understanding how rising global temperature
translates to risks for society and natural ecosystems is critical
to prepare for, and strive to reduce, the scale of impacts. But
predicting consequences for different regions is difficult because
while global temperature is a good indicator of global change,
local impacts can be much more pronounced, Levermann


A comparison of climate impacts across the world
by the end of the century under RCP2.6 and RCP8.5. (a) Average
surface temperature, (b) average annual rainfall, (c) Arctic sea
ice extent and (d) change in ocean pH. Source:
IPCC 5th Assessment Report

A continuum, not a precipice

For international climate policy purposes, it
makes sense to think in terms of the climate damages expected at
different degrees of warming.

Two degrees is an appropriate middle ground
between what we can no longer avoid and the level of further risk
we’re willing to accept, Levermann suggests.

“We can’t really keep to
one degree target anymore … At three degrees warming, Greenland is
going to vanish and corals are going to be largely extinct… I would
personally argue three degrees is too much and one degree is no
longer achievable so two degrees is a reasonable target, but that
is for society to decide.”

But setting two degrees as a boundary into
“dangerous” climate change only works as a political target if its
understood as a point along a continuum, not as a climate
precipice, Levermann warns.

In other words, failing on the two degree target
doesn’t mean we should all give up and go home. But admitting
defeat means accepting a greater level of risk – and at that point
preventing temperatures straying too far above two degrees should
be paramount.

As to what counts as unacceptably high risk,
that comes down to a judgement call, Betts concludes:

“How much these changes
‘matter’ or not is largely a matter of personal values and ethics …
[W]e have to judge whether we think the benefits to ourselves are
worth the risks to other species or future generations of our

Only a few years on since countries agreed on
two degrees as an appropriate level of climate ambition, it’s
important to remember the science that underpins the agreement. As
Professor Rowan Sutton told a Royal Society meeting this week,
decision-making on climate comes down to our appetite for risk. Any
decision to expose ourselves to higher climate risk should at least
be a conscious and deliberate one, if not necessarily a prudent