Model–data comparison of climate changes in the future and during the LIG. a, RCP2.6 model ensemble (CCSM4) results of mean annual surface air temperature (SAT) anomalies for the time interval 2080–2099 relative to our pre-industrial reference interval, 1850–1900. b, Observed LIG (125 ka) annual sea surface temperature (SST) anomalies relative to its reference period of 1870–1889 (dots) overlain on top of CCSM3 SAT anomalies for the 125 ka time window relative to 1850. White areas in polar regions in panels a and b represent the modelled sea ice extent. Graphic: Fischer, et al., 2018 / Nature Geoscience

5 July 2018 (ARC Centre of Excellence for Climate System Science) – Future global warming may eventually be twice as warm as projected by climate models under business-as-usual scenarios and even if the world meets the 2°C target sea levels may rise six metres or more, according to an international team of researchers from 17 countries.The findings published last week in Nature Geoscience are based on observational evidence from three warm periods over the past 3.5 million years when the world was 0.5°C-2°C warmer than the pre-industrial temperatures of the 19th Century.The research also revealed how large areas of the polar ice caps could collapse and significant changes to ecosystems could see the Sahara Desert become green and the edges of tropical forests turn into fire-dominated savanna.“Observations of past warming periods suggest that a number of amplifying mechanisms, which are poorly represented in climate models, increase long-term warming beyond climate model projections,” said lead author, Prof Hubertus Fischer of the University of Bern.“This suggests the carbon budget to avoid 2°C of global warming may be far smaller than estimated, leaving very little margin for error to meet the Paris targets.”To get their results, the researchers looked at three of the best-documented warm periods, the Holocene thermal maximum (5000-9000 years ago), the last interglacial (129,000-116,000 years ago) and the mid-Pliocene warm period (3.3-3 million years ago).The warming of the first two periods was caused by predictable changes in the Earth’s orbit, while the mid-Pliocene event was the result of atmospheric carbon dioxide concentrations that were 350-450ppm – much the same as today.Combining a wide range of measurements from ice cores, sediment layers, fossil records, dating using atomic isotopes, and a host of other established paleoclimate methods, the researchers pieced together the impact of these climatic changes.In combination, these periods give strong evidence of how a warmer Earth would appear once the climate had stabilized. By contrast, today our planet is warming much faster than any of these periods as human caused carbon dioxide emissions continue to grow. Even if our emissions stopped today, it would take centuries to millennia to reach equilibrium.The changes to the Earth under these past conditions were profound – there were substantial retreats of the Antarctic and Greenland ice sheets and as a consequence sea-levels rose by at least six metres; marine plankton ranges shifted reorganising entire marine ecosystems; the Sahara became greener and forest species shifted 200 km towards the poles, as did tundra; high altitude species declined, temperate tropical forests were reduced and in Mediterranean areas fire-maintained vegetation dominated.“Even with just 2°C of warming – and potentially just 1.5°C – significant impacts on the Earth system are profound,” said co-author Prof Alan Mix of Oregon State University.“We can expect that sea-level rise could become unstoppable for millennia, impacting much of the world’s population, infrastructure, and economic activity.”Yet these significant observed changes are generally underestimated in climate model projections that focus on the near term. Compared to these past observations, climate models appear to underestimate long term warming and the amplification of warmth in Polar Regions.“Climate models appear to be trustworthy for small changes, such as for low emission scenarios over short periods, say over the next few decades out to 2100. But as the change gets larger or more persistent, either because of higher emissions, for example a business-as-usual-scenario, or because we are interested in the long term response of a low emission scenario, it appears they underestimate climate change.,” said co-author Prof Katrin Meissner, Director of the University of New South Wales Climate Change Research Centre.“This research is a powerful call to act. It tells us that if today’s leaders don’t urgently address our emissions, global warming will bring profound changes to our planet and way of life – not just for this century but well beyond.”

Global warming may be twice what climate models predictImpacts and responses of components of the Earth system to global warming, during the Holocene thermal maximum (HTM), the Last Interglacial (LIG), and the mid-Pliocene warm period. Graphic: Fischer, et al., 2018 / Nature Geoscience

ABSTRACT: Over the past 3.5 million years, there have been several intervals when climate conditions were warmer than during the pre-industrial Holocene. Although past intervals of warming were forced differently than future anthropogenic change, such periods can provide insights into potential future climate impacts and ecosystem feedbacks, especially over centennial-to-millennial timescales that are often not covered by climate model simulations. Our observation-based synthesis of the understanding of past intervals with temperatures within the range of projected future warming suggests that there is a low risk of runaway greenhouse gas feedbacks for global warming of no more than 2 °C. However, substantial regional environmental impacts can occur. A global average warming of 1–2 °C with strong polar amplification has, in the past, been accompanied by significant shifts in climate zones and the spatial distribution of land and ocean ecosystems. Sustained warming at this level has also led to substantial reductions of the Greenland and Antarctic ice sheets, with sea-level increases of at least several metres on millennial timescales. Comparison of palaeo observations with climate model results suggests that, due to the lack of certain feedback processes, model-based climate projections may underestimate long-term warming in response to future radiative forcing by as much as a factor of two, and thus may also underestimate centennial-to-millennial-scale sea-level rise.

Palaeoclimate constraints on the impact of 2 °C anthropogenic warming and beyond