Helmholtz-Zentrum Geesthacht, 2018-02-19
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  • Are we heading towards 4-degrees warming? And if yes, should we be concerned?

    By The Earth League members Johan Rockström, Hans Joachim Schellnhuber, Mario Molina, Brian Hoskins, Guy Brasseur, Carlos Nobre, Peter Schlosser, Veerabhadran Ramanathan, Nebojsa Nakicenovic, Youba Sokona, Leena Srivastava, Jennifer Morgan

    © iStock © iStock

    The body of scientific evidence indicating that our civilization has already caused significant global warming is overwhelming: Of the almost 14 000 peer-reviewed climate-change articles from the last two decades, analyzed recently by the geologist James Powell, only 24 deny the warming or its human cause. Also, research keeps piling up studies demonstrating that even bigger modifications of the world’s environment can be expected if the release of heat-trapping gases from industry, agriculture, transportation and settlements is not curbed immediately: New projections from the Coupled Model Intercomparison Project (CMIP5) result in up to 6°C higher planetary temperatures by 2100 (as compared to pre-industrial levels) under the assumption of no mitigation. Finally, a disturbing picture of the negative impacts and dramatic risks accompanying unbridled warming of the Earth is emerging: According to a model calibrated with data from the past millennium, a warming by “only” 2°C would generate, by 2300, a sea-level rise of 2.7m above the current value - to name just one dire consequence.

    In response to these alarming facts and figures, crucial political institutions (such as the UN Security Council), international agencies (such as the IMF) and business associations (such as the WBCSD) conclude that the long-term prosperity of the human enterprise is challenged. Therefore, these eminent stakeholders call for aggressive climate stabilization, regardless of residual cognitive uncertainties that may never be entirely eliminated. Global institutions have recognized that science speaks effectively with one voice about the reality of climate change. A recent flagship report by the World Bank epitomizes this new dynamics. The document is based on a commissioned review of the latest scientific literature, performed by the Potsdam Institute (PIK) and Climate Analytics (CA), both based in Germany. The World Bank was particularly interested in the most evident consequences that nature and civilization would have to face in a 4°C warmer world. And the Bank wanted to learn how such a change in the planetary environment might impinge on their strategies for world-wide human development and poverty reduction.

    PIK and CA fulfilled their task similarly to how the IPCC performs its assessments: The entire body of pertinent studies was searched by about 15 experts, with a strong focus on the peer-reviewed literature in the best international journals. Thus, the report drew upon the inputs from about 1500 international scholars. In order to double-check the scientific quality of the collected information, the resulting piece in turn was peer-reviewed by 13 renowned scientists nominated by the World Bank. This procedure provided the best possible reflection of the relevant state of the climate-change art. Let us now turn to the second overarching question raised in the title. Is a 4-degrees warming a problem? The answer reads, with marginal uncertainty, “yes”. In fact, such a future can only be responsibly described as confronting the world with a potentially disastrous, and thus unacceptable, prospect. This environmental sea-change would most likely cause massive stresses and disruptions in numerous natural and socioeconomic systems across the world. Some of these impacts would neither be acceptable nor manageable for the people directly affected, most of them living in the tropics and sub-tropics. The World Bank report lists several of the most deleterious effects projected and explains why the poor countries would be hit hardest. Much of this has to do with what scientists call “variability”.

    In order to appreciate the argument, let us take a long look back on the making of our civilization. This happened over the past 10 000 years, a period of unprecedented stable and temperate climate conditions, which allowed our ancestors to settle down for good. During that unique window of cultural opportunity, the planetary mean surface temperature oscillated only plus/minus one degree around a value of approximately 14 °C, so the global population could grow from a few millions to 7 billion at present.

    Ironically, the human success story implies that its underlying climate stability eventually turns self-destructive: Billions of people burn enormous amounts of fossil fuels and thereby enhance the natural greenhouse effect. Mostly as a consequence of this, the Earth’s surface temperature has risen by roughly 0.8°C since 1900. This seems a small amount, but it was already sufficient to drive climate variability away from its historical range. A recent study concludes that local temperature records meanwhile occur five times more often than would have been the case without global warming. The last two decades were also punctuated by devastating floods (like the Pakistan deluge in 2010) that may be related to an incipient restructuring of the atmospheric circulation. The signs on the climate wall as expressed by the accelerated melting of Arctic sea ice and by the retreat of the overwhelming majority of glaciers worldwide are there for all to see.

    Yet this is just the beginning: We have strong evidence (as documented in the 4th IPCC Report from 2007) that a 2°C warming of the planet would imply major disruptions in the world’s ecosystems, high risks to rain-fed agriculture in tropical regions, and sea-level rise threatening the very existence of low-lying island states and coastal regions. For example, without accounting for thermal adaptation options around 90% of the Earth’s coral reef cells are projected to be at risk of long-term degradation at a global warming level of 1.5 °C.

    Now let us inspect a 4-degrees future. Palaeoclimatic studies indicate that large parts of the world as we know it would eventually disappear. Sea levels would rise (slowly, but inexorably) by at least 10-20 meters; many sub-tropical regions (but also parts of the Mediterranean) would turn into inhabitable deserts, so massive population movements to nations in the North are likely in response; and the 6th “Great Extinction” would drastically diminish global biodiversity and the ecosystem services that depend on it. The World Bank report provides crucial hints for this assessment, many of them related to climate variability. For instance, in tropical regions temperatures in more than 90% of the months in 2080-2100 are expected to lie further than 5 standard deviations above the traditional temperature mean; under natural variability such events would occur with a probability of less than 0.00003 percent!

    © Nasa © Nasa

    Moreover, it is critical to realize that global warming might not stop at the 4-degrees level. Powerful feedback processes that very likely will push the warming even higher (such as methane release from thawing permafrost domains and peat bogs on land as well as from huge frozen layers on the sea floor) could be set into irreversible operation. And, 4-degrees planetary warming means some 8-degrees change close to the Arctic, which will cause even larger impacts on the Eurasian and North-American land mass and the surrounding seas. So, much depends on the answer to our first question, is a global mean temperature rise by 4°C possible or even likely? Unfortunately, the latest research supports a clear “yes” again. A recent study published in Nature Climate Change concluded that if we continue to churn out greenhouse gases at the current rate, we face the risk of generating 4.2-5 °C planetary warming by the end of this century.

    Other recent scientific studies provide evidence that humankind, if it does not veer off its current track, runs a non-negligible risk of suffering from the 4-degrees shock even before 2100. Although climate science only tells us what might happen and not what to do about it, we authors feel that this is an unacceptable prospect. Nations go to war, implement mass vaccinations of their populations, and organize expensive insurance and security systems (such as anti-terror measures) to address much fainter threats. However, our societies seem to be willing to impose immense risks on future generations.

    This sarcastic statement fits the current public debate about climate change all too well. There are some who say that humanity could not cause a 4-degrees warming by any conceivable means; others hasten to declare environmental defeat by maintaining that the international climate-policy goal of confining the planetary warming to less than 2 degrees is already a lost case. Yet the scientific evidence is just what it is. And it tells us that both these speculations are wrong. Regardless of the sometimes made (erroneous) claim that global warming has already stopped, evidence is that once well-known impacts from El Niño, volcanic aerosols and solar variability are removed from the observations, the warming trend of the ocean-atmosphere system is unbroken; and that it will continue (potentially towards 4°C) unless serious mitigation action is taken. That global warming continues unabated over the last decade is confirmed by ocean measurements. Ninety percent of the additional heat that the Earth system absorbs due to the increase in greenhouse gases is stored in the oceans, and the global array of thousands of scientific measurement robots in the oceans proves that they keep heating up at a steady pace. Meanwhile satellites show that sea levels also keep rising steadily.

    Also, there is ample evidence that we can hold the 2°C-line indeed. The recent “Global Energy Assessment” shows that a rapid world-wide transformation to sustainable energy access for all without fossil fuels is technically and economically feasible. This is demonstrated, for instance, in Germany, the second-biggest export country on the globe, where the so-called “Energiewende” – even though still on fragile grounds - is on the move: green feed-in tariffs as well as sell-and-buy markets for energy down to the household level have resulted in massive expansion of renewables. Current trends indicate that a staggering share of up to 50% of the country’s electricity could come from wind, solar, hydro and biomass by as early as 2020. This is also demonstrated in Brazil, where 45% of its total energy use is renewable and over 80% of Brazil's electricity (hydro, biomass, and wind). In addition, mitigating emissions of four climate warming pollutants (Methane; Black Carbon; Ozone and HFCs) using just available technologies can cut the rate of warming in the coming decades by as much as 50% and reduce end-of-century sea-level rise by about 30%.

    © The World Bank © The World Bank

    Science and technology are rising to the challenge of finding clever solutions for the “Great Transformation” towards global sustainability. An international mobilization of the most advanced international research on climate change, biodiversity, land, oceans, and governance, is being launched this year. A vanguard capacity for this program will be the just-established “Earth League”, a cosmopolitan alliance of some of the world’s leading scientists and finest research institutions addressing the grand challenges of global change and human development.

    In 2015, the global community wants to seal a ground-breaking climate agreement and also replace the rather conventional Millennium Development Goals by integrated Sustainable Development Goals. Two years is a very short time for creating the necessary political momentum. Yet the scientific consensus has grown exceedingly strong to warrant action.

    Download the full World Bank Report I


    Download the full World Bank Report II


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