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Scientists compare climate change impacts at 1.5C and 2C

Posted on 2 June 2016 by Guest Author

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

Half a degree makes a very big difference when judging how different parts of the world will feel the effects of climate change.

This is the conclusion from the first study to compare and contrast the consequences of 1.5C world compared to a 2C world, published today in Earth System Dynamics.

Both 2C and 1.5C are explicitly mentioned in the Paris agreement as potential upper limits for global warming since the preindustrial era, but details from scientists on how the temperature thresholds compare have been sparse.

For example, an extra 0.5C could see global sea levels rise 10cm more by 2100, water shortages in the Mediterranean double and tropical heatwaves last up to a month longer. The difference between 2C and 1.5C is also “likely to be decisive for the future of coral reefs”, with virtually all coral reefs at high risk of bleaching with 2C warming.

The authors presented their research today at the European Geosciences Union, an annual major gathering of geoscientists taking place this week in Vienna.

“Two-headed goal”

The Paris agreement – adopted in December 2015 and due to be officially signed by more than 150 countries on Friday – codified what the authors of today’s study call a “two-headed” temperature goal.

It pledged to keep the average global surface temperature “well below 2C” and “pursue efforts” to limit the increase since preindustrial times to 1.5C.

The nod to 1.5C recognised that many low lying island nations are already feeling the impacts of climate change and that coral reef and Arctic ecosystems face high risks well below 2C.

But the specific reference to 1.5C as well as 2C caught the scientific community somewhat off-guard. Today’s paper says:

“Despite the prominence of these two temperature limits, a comprehensive overview of the differences in climate impacts at these levels is still missing.”

A recent commentary in Nature by Prof Simon Lewis, professor of global change at University College London, is a little stronger on this point. As he puts it:

“The emergence of 1.5 C as a serious policy position comes with important lessons for scientists. The global research community has shockingly little to say on the probable impacts of a 1.5 C rise.”

The scientific community now, at least, seems to be rising to the challenge. Last week, the IPCC confirmed it will dedicate one of its special reports to the 1.5C goal. This is due to be published in 2018.

Work on today’s paper began in 2014, long before the Paris conference. The United Nations Framework Convention on Climate Change called on scientists to explore the difference between a 1.5C and 2C long term goal, as part of its 2013-2015 review.

Prof Carl-Friedrich Schleussner, scientific advisor at Climate Analytics in Germany and lead author of today’s study, tells Carbon Brief:

“[The review] concluded last year that, while 2C cannot be considered safe and 1.5C would clearly be a safer limit, the science on 1.5C is less robust than on 2C. So clearly, there’s a research gap here.”

A 1.5C vs 2C world

The study compared how extreme weather, water availability, crop yields, sea level rise and risks to coral reefs differ in a world where global temperature rises 1.5C, compared to if it rises 2C.

Using 11 climate models, the authors looked at how each of the impacts plays out globally, as well as in 26 different regions. This is important since the world won’t warm at the same pace everywhere, the paper notes.

Infographic: How do the impacts of 1.5C of warming compare to 2C of warming?

Infographic: How do the impacts of 1.5C of warming compare to 2C of warming? By Rosamund Pearce for Carbon Brief.

Some of the most dramatic differences occur with heat extremes, with heatwaves in the tropics lasting up to three months with 2C warming, compared to two months with 1.5C. The paper says:

“[T]he additional 0.5C increase in global-mean temperature marks the difference between events at the upper limit of present-day natural variability and a new climate regime, particularly in tropical regions.”

High northern latitudes are expected to see some of the biggest increases in heavy rainfall, with the maximum over a five-day period rising by 7% for 2C warming, compared to 5% for 1.5C.

At the same time, water scarcity in the Mediterranean is likely to be twice as severe at 2C than at 1.5C, with climate-induced shortfalls of 17% compared to 9% (relative to 1986-2005 levels).

The study shows global sea level rising 50cm by 2100 with 2C of warming, compared to 40cm for 1.5C (both relative to 2000). Warming of 2C would also put 98% of the world’s reefs at risk of coral bleaching from 2050 onwards, compared to 90% for 1.5C.

Global changes in the intensity of hot extremes (left) and the duration of warm spells (right) with 2C warming (top), 1.5C (middle) and the difference between the two (bottom) Source: Schleussner, C-F, et al., (2016)

Global changes in the intensity of hot extremes (left) and the duration of warm spells (right) with 2C warming (top), 1.5C (middle) and the difference between the two (bottom) Source: Schleussner, C-F, et al., (2016)

Climate change impacts on crops are complicated, depending a lot on the crop in question and where in the world you’re looking. Schleussner tells Carbon Brief:

“However, observational evidence is pointing towards the fact that climate change is already negatively impacting agricultural yields.”

The new study’s results are in line with previous research suggesting an average reduction of about 6% in global wheat yields per degree of warming, Schleussner tells Carbon Brief.

Tropical regions, such as West Africa, Southeast Asia, Central America and northern South America, are likely to be worst affected, the study notes, with yields of wheat and maize set to decline.

The study accounts for the potential of higher CO2 to have a positive effect on crop yields, particularly in high-latitude regions, but notes this is still not well understood.

Reality check

The sheer scale of the contrast between climate impacts 1.5C and 2C came as “quite a surprise” to the authors, Schleussner tells Carbon Brief:

“Our results clearly show that the difference between 1.5C and 2C is not only a matter of gradual change.”

So, should 2C remain the reference point for “dangerous anthropogenic interference in the climate system” or are impacts at 1.5C severe enough to warrant a rethink?

While science can make sure there’s as much evidence as possible, that’s a conversation that extends further than the scientific community, say the authors in the paper:

“Although the assessment of levels of dangerous interference is primarily a political process that requires value judgements and depends on different world views, it needs to be informed by the best available science outlining the impacts of climate change and mitigation efforts.”
Carbon budget countdown infographic for different levels of warming

How many years of current emissions would use up the IPCC’s carbon budgets for different levels of warming?

Acknowledging that capping warming at 1.5C is preferable to 2C is one thing, but it’s important to talk about the realities of getting there. And the scale of the challenge is immense.

On current emissions, the carbon budget for 1.5C will effectively be blown in about four and a half years, as our graphic above shows (Note: figures are based on 2014 emissions).

The consequence of this is that any realistic possibility of limiting warming to 1.5C in the long term means overshooting the target and somehow coming back down. Schleussner tells Carbon Brief:

“Scientific findings…show that it is both physically and economically feasible to limit warming to below 1.5°C by 2100, after temporarily exceeding 1.5°C in the 2050s (but still staying well below 2°C)."

But doing so involves relying on being able to “suck” carbon dioxide out of the air, using so-called negative emissions technologies (NETs). Carbon Brief ran a special series of article last week looking in-depth at possible approaches and how feasible experts think they are.

Even if overshooting and coming back down to 1.5C were technically possible, there’s no guarantee the consequences for ecosystems would be the same as if we hadn’t cross the 1.5C boundary at all. Whether or not climate change impacts are “reversible” is a very important research topic right now, says Schleussner.

Today’s study is an important first step to understanding the real-world consequences of what countries agreed to do, in principle, in Paris.

While it lays out the scientific reasoning behind a 1.5C target, the question of if and how we get there is part of a far bigger conversation, one that’s particularly pertinent as nations gather in New York this week to reaffirm their collective commitment.

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Comments 1 to 6:

  1. Further distinctions between a 1.5C and 2.0C future are essential, especially regarding the 100x more powerful greenhouse gas methane which has been naively excluded from IPCC prediction models. Thank you.

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  2. These predictions ignore mass changes just as early estimates of ice sheet melting ignored the side effects of faster ice flows into the sea and only considered thermodynamics.  If the Arctic Ocean becomes an area of prevailing rising air as the ocean warms, it should reverse the Polar Hadley cell and suck climate zones northward rather suddenly.  In such an eventuality, all bets are off and we have sudden extensive crop failures in the Northern Hemisphere.  Have you noticed that in the Mana Loa Carbon dioxide web site that CO2 from April 2015to April2016 is 4ppm.  While this may be an effect of El Nino and the line on the graph will revert to the steady 2 to 2.5 yearly increase, it could also signal that one or more carbon sinks is shutting down.  If so, the outlook makes redundant all studies such as this.

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  3. "Both 2C and 1.5C are explicitly mentioned in the Paris agreement as potential upper limits for global warming since the preindustrial era," is in the article. There can be no limit to global warming (or ocean acidification and warming despite the views freely expressed. The greenhouse emissions have already put in train an irreversible process. Reducing the rate of emissions will only slow down the rate of warning slightly with absorption in the oceans continuing to impact on the atmospheric concentration level.

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  4. Professor Kevin Anderson presentation on this

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  5. The IPCC assessments have been projecting the impacts of 1ºC to 5ºC since the Third Assessment in 2001 at least. The 2007 AR4 made it totally obvious that impacts above 1ºC will be highly dangerous, >1.5ºC disastrous, and >2ºC catastrophic. The Stern Commission in 2006 also included projected impacts of 1-5ºC. Given that 2ºC was never a scientific limit, and that the most vulnerable nations practically begged for a 1.5ºC limit (and some a 1ºC limit) in Copenhagen in 2009 (ahem, six years ago), why has 1.5ºC come as such a surprise to climate change scientists? 

    I just read a quote from Slavoj Žižek, a Slovenian philosopher: "Love is what makes sex more than masturbation." I am not feeling the love from scientists these days, if you get my drift. If +2ºC is really super bad, and +1.5ºC is only slightly less bad, why on Earth do scientists need "a comprehensive overview of the differences in climate impacts at these levels" or more "robust" science on 1.5?

    What, pray tell, is keeping climate change scientists from declaring their love and getting on with the task of helping us attain zero carbon emissions?

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  6. GreenHearted@5,
    Hopefully you are not expecting climate scientists to 'create a solution'.

    Climate scientists develop and provide advancements of information and understanding (including clearer understanding of the likely consequences of 1.5C and 2C warming).

    All of the world's wealthiest people and political leaders are the ones to demand a solution from.

    As a minimum there should be powerful popular demands for all of those wealthy powerful supposed leaders of global humanity to declare that the only legitimate people with wealth or power are the ones who can prove their actions have promoted the advancement of humanity to a lasting better future for all of humanity into the almost eternal future humanity could enjoy on this amazing planet.

    The lack of popular demand for that type of leadership is the real problem.

    A related problem is that getting away with less acceptable behaviour is almost always more profitable and cheaper (the one getting away with being least acceptable has a temporary competitive advantage, temporary for as long as they can get away with it). That fact makes it easier to drum up popular support for understood to be less acceptable behaviour than it is to gain support for more responsible behaviour that develops a gift of a better future for the benefit of future generations.

    One of the most galling evaluations performed by supposed leaders pretending to look like they care about this issue is the comparison of the perceived lost opportunity for the wealthy in the current generation to avoid imposing costs on future generations. The objective of such evaluations is to create the impression that it is OK to impose costs on future generations if the future costs as evaluated by today's generation will be equal to or less than the perceived lost opportunity of today's generation of greedy callous opportunists (which is obviously unacceptable when it is actually thought about - negative impacts on future generations are inexcusable, regardless of temporary developed tastes for regional popular support of such clearly unacceptable actions).

    So climate scientists are not responsible for coming up with a solution. Leaders need to be required to prove they are pursuing valid solutions - proven valid based on the constantly improved best understanding of what is going on and how to advance humanty to a lasting better future for all.

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