What happens if we overshoot the two degree target for limiting global warming?

This is a re-post from Carbon Brief by Roz Pidcock

Two degrees is the internationally-agreed target for limiting global warming, and has a long history in climate policy circles. 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 limit.

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 risks

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

Global temperature has risen 0.85 degrees Celsius since 1880, according to the latest report 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 estimate.

AR5_SPMTemp Since 1850

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 of extreme weather.In Europe, heatwaves like the 2003 event which killed 70,000 people are already ten times more likely 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 estimate.

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 manage.

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 below).

(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 line).

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 Levermann:

"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 us:

"[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 get:

"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 Celsius"

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 says.


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 own."

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 one.  

Posted by Guest Author on Thursday, 18 December, 2014

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