The primary global average surface temperature (GAST) estimate (using 61 proxy reconstructions) is plotted as a function of time, with the median in black and the 95 percent interval in grey. The GAST estimation method is repeated for a clustering of the data (11 clusters and 18 individual reconstructions), with the median shown in cyan, and for only the 5 proxy reconstructions that cover the past 2 Myr, with the median shown in orange. Graphic: Snyder, 2016 / Nature

WASHINGTON, 26 September 2016 (AP) – A new study paints a picture of an Earth that is warmer than it has been in about 120,000 years, and is locked into eventually hitting its hottest mark in more than 2 million years. As part of her doctoral dissertation at Stanford University, Carolyn Snyder, now a climate policy official at the U.S. Environmental Protection Agency, created a continuous 2 million year temperature record, much longer than a previous 22,000 year record. Snyder’s temperature reconstruction, published Monday in the journal Nature, doesn’t estimate temperature for a single year, but averages 5,000-year time periods going back a couple million years. Snyder based her reconstruction on 61 different sea surface temperature proxies from across the globe, such as ratios between magnesium and calcium, species makeup and acidity. But the further the study goes back in time, especially after half a million years, the fewer of those proxies are available, making the estimates less certain, she said. These are rough estimates with large margins of errors, she said. But she also found that the temperature changes correlated well to carbon dioxide levels. Temperatures averaged out over the most recent 5,000 years – which includes the last 125 years or so of industrial emissions of heat-trapping gases – are generally warmer than they have been since about 120,000 years ago or so, Snyder found. And two interglacial time periods, the one 120,000 years ago and another just about 2 million years ago, were the warmest Snyder tracked. They were about 3.6 degrees warmer than the current 5,000-year average. With the link to carbon dioxide levels​ and taking into account other factors and past trends, Snyder calculated how much warming can be expected in the future. Snyder said if climate​ factors are the same as in the past – and that’s a big if – Earth is already committed to another 7 degrees or so of warming over the next few thousand years. “This is based on what happened in the past,” Snyder said. “In the past it wasn’t humans messing with the atmosphere.” [more]

Study: Earth now the warmest it’s been in 120,000 years

ABSTRACT: Reconstructions of Earth’s past climate strongly influence our understanding of the dynamics and sensitivity of the climate system. Yet global temperature has been reconstructed for only a few isolated windows of time1, 2, and continuous reconstructions across glacial cycles remain elusive. Here I present a spatially weighted proxy reconstruction of global temperature over the past 2 million years estimated from a multi-proxy database of over 20,000 sea surface temperature point reconstructions. Global temperature gradually cooled until roughly 1.2 million years ago and cooling then stalled until the present. The cooling trend probably stalled before the beginning of the mid-Pleistocene transition3, and pre-dated the increase in the maximum size of ice sheets around 0.9 million years ago4, 5, 6. Thus, global cooling may have been a pre-condition for, but probably is not the sole causal mechanism of, the shift to quasi-100,000-year glacial cycles at the mid-Pleistocene transition. Over the past 800,000 years, polar amplification (the amplification of temperature change at the poles relative to global temperature change) has been stable over time, and global temperature and atmospheric greenhouse gas concentrations have been closely coupled across glacial cycles. A comparison of the new temperature reconstruction with radiative forcing from greenhouse gases estimates an Earth system sensitivity of 9 degrees Celsius (range 7 to 13 degrees Celsius, 95 per cent credible interval) change in global average surface temperature per doubling of atmospheric carbon dioxide over millennium timescales. This result suggests that stabilization at today’s greenhouse gas levels may already commit Earth to an eventual total warming of 5 degrees Celsius (range 3 to 7 degrees Celsius, 95 per cent credible interval) over the next few millennia as ice sheets, vegetation, and atmospheric dust continue to respond to global warming.

Evolution of global temperature over the past two million years