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Shallow bleaching corals, split level with the island, Lissenung, New Ireland, Papua New Guinea,
© Jurgen Freund/Nature Picture Library/Corbis
21 April 2016 13:40

Scientists compare climate change impacts at 1.5C and 2C

Roz Pidcock

Roz Pidcock

Roz Pidcock

Roz Pidcock

21.04.2016 | 1:40pm
Global temperatureScientists compare climate change impacts at 1.5C and 2C

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.

Main image: Shallow bleaching corals, split level with the island, Lissenung, New Ireland, Papua New Guinea, June 2010
Sharelines from this story
  • Scientists compare climate change impacts at 1.5C and 2C
  • cosmicomics

    Based on recent research on melting in Greenland and Antarctica, and based on observations of accelerating sea level rise, it seems to me that the 2100 SLR projections are much too low.

  • Robin_Guenier

    You say that the Paris agreement “pledged to keep the average global surface temperature “well below 2C”“.

    Unfortunately, that isn’t accurate. The relevant provision, Article 2.1.(a), refers to an aim, not a pledge. In other words, it’s no more than an aspiration. And it’s an aspiration that has to be read “in the context of sustainable development and efforts to eradicate poverty, …” – a reference back to the 1994 UN Framework Convention on Climate Change that allows developing countries (responsible for about 70 percent of global emissions) to give those issues overriding priority. Moreover, under Article 4.4, of the Paris text, developing countries are merely “encouraged to move over time towards economy-wide emission reduction or limitation targets …

    Rather than being concerned about the difference between 1.5ºC and 2ºC, perhaps we should be focusing on the consequences of seriously overshooting the 2ºC target.

    The Paris text can be found here:

  • Timmons Roberts

    This is vitally important thinking. The task now is to develop a credible pathway to avoid 1.5 degrees. We transformed our economies in months back in 1941. We can again.

  • john

    Just how to give the information and have the general public understand that is the question.
    It seems no amount of studies, which all point to the same evidence, is not of any use, if the vast majority do not know.
    I suppose being realistic if the most powerful media outlets are beamed into most houses every day and the message is the opposite there is not exactly much hope.
    Will the actions of China in trying to reduce its carbon footprint be taken into note to spur some western democracy’s into action?
    The Western European countries seem to be on a course of at least trying.

    • Robin_Guenier

      China’s actions are aimed at tackling the appalling problem of urban pollution. There’s no real evidence that the Chinese politburo thinks that “trying to reduce its carbon footprint” is a priority – or even an important objective.

      Your comment about the need “to spur some western democracies into action”, while noting that Western European countries are trying indicates you think the US is not doing enough. Is that your view?

      • john

        The USA does have some credit overall as CO2 emissions have been reduced in some sections of the industrial area.
        Yes there is a very good 9 part series, which covers particulate emissions in China.
        Childhood without seeing white clouds and stars
        Putting the above into google will find the series.
        Because of the bad health outcome Chinese leadership have been forced to act.
        England and the USA did this quiet a long time ago to remove the smog quaintly called haze in China.
        India is also facing the same problems and will be forced to act.
        I do not think there is sufficient political agreement to change long term outcomes mainly caused by the short election cycles.
        Implementing long term plans where some other person later on can claim credit is not the way the adversary political system works unfortunately.
        When major parties come together to implement actions together then there will be outcomes on a very broad scale.

  • GeoffBeacon

    I originally missed this article but found it through Neven’s Arctic Forum.

    I’ve just redone some similar calculations (Following the Carbon Brief piece in November 2014) as a postscript to “Is green growth a fantasy?” ( ).

    Here there may be two things missing. Firstly the carbon budget should be reduced to account for greenhouse gasses other than CO2. The World Resources Institute says ( ):

    “one can argue for an even smaller budget and additional emissions constraints because non-CO2 gases are not included in 1 trillion tonne C figure. For example, short-lived greenhouse gases, such as methane, are not included in – nor necessarily appropriate for – the 1 trillion tonne C budget approach because they play a secondary role in influencing long-term warming.

    However, when non-CO2 forcings are taken into account, the budget is reduced and that budget may depend on the scenario studied. For example, according to one scenario studied in the IPCC AR5 (RCP 2.6), when non-CO2 greenhouse gases are considered, the budget drops much lower to 790 PgC.”

    That means that the effect of other greenhouse gasses reduces the overall budget to 79% of the original. The remaining carbon budgets are measured in terms of CO2 so, as a rough estimate they should be reduced by roughly 21%. This now gives…

    The remaining carbon budget for a 66% chance of avoiding 1.5˚C becomes …

    21 tonnes CO2 per person: 4 years to 1.5˚C

    The remaining carbon budget for a 66% chance of avoiding 2.0˚C becomes…

    85 tonnes CO2 per person: 16 years to 2.0˚C

    The other issue is the “missing feedbacks” in the CMIP5 climate models, which consequently may have overestimated the budgets. I am waiting for a promised reply from DECC on this.

  • I attended a talk by Dr Joeri Rogelj this week. He has been one of the few researchers closely examining the 1.5C pathways available to us. They all involve temperature overshoot and negative emissions technologies that are yet to be developed. They also involve us taking rapid early action. The stronger and earlier the emissions abatement through energy efficiency, demand management, and renewables, the less we will need to rely on BECCS to bring CO2 down later.
    See my blog post Is #1o5C #ParisAgreement temperature pathway possible? and my storify curation of tweets.

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