Earth's Average Temperature Lower Now Than It Was 5,000 Years Ago

That's the conclusion of a new study, "A Reconstruction of Regional and Global Temperature for the past 11,300 Years," being published in the journal Science today. But before drawing in a sigh of relief about the future of global warming, the researchers also point out that the rapid warming over the last century has essentially cancelled out 2,000 years of gradual cooling.

The researchers from Oregon State University and Harvard University came to their results by combining 73 different proxy climate records (assembled into what they call stacks) spanning the past 11,500 years. They report:

Our results indicate that global mean temperature for the decade 2000–2009 has not yet exceeded the warmest temperatures of the early Holocene (5000 to 10,000 yr B.P.). These temperatures are, however, warmer than 82% of the Holocene distribution as represented by the Standard5×5 stack, or 72% after making plausible corrections for inherent smoothing of the high frequencies in the stack. In contrast, the decadal mean global temperature of the early 20th century (1900–1909) was cooler than >95% of the Holocene distribution under both the Standard 5×5 and high-frequency corrected scenarios. Global temperature, therefore, has risen from near the coldest to the warmest levels of the Holocene within the past century, reversing the long-term cooling trend that began ~5000 yr B.P.

From the abstract:

Surface temperature reconstructions of the past 1500 years suggest that recent warming is unprecedented in that time. Here we provide a broader perspective by reconstructing regional and global temperature anomalies for the past 11,300 years from 73 globally distributed records. Early Holocene (10,000 to 5000 years ago) warmth is followed by ~0.7°C cooling through the middle to late Holocene (<5000 years ago), culminating in the coolest temperatures of the Holocene during the Little Ice Age, about 200 years ago. This cooling is largely associated with ~2°C change in the North Atlantic. Current global temperatures of the past decade have not yet exceeded peak interglacial values but are warmer than during ~75% of the Holocene temperature history. Intergovernmental Panel on Climate Change model projections for 2100 exceed the full distribution of Holocene temperature under all plausible greenhouse gas emission scenarios.

The new study finds that changes in the amount of summertime sunlight striking the Northern Hemisphere due to changes in the Earth's orbital orientation toward the sun is chiefly responsible for the recent alternation between Ice Ages and warmer periods like the one we're currently in. From the study's press release:

"During the warmest period of the Holocene, the Earth was positioned such that Northern Hemisphere summers warmed more," [Shaun] Marcott, [lead author from OSU] said. "As the Earth's orientation changed, Northern Hemisphere summers became cooler, and we should now be near the bottom of this long-term cooling trend – but obviously, we are not."

So how do recent changes in global average temperature compare to the past record of climate? Again from the press release:

"The last century stands out as the anomaly in this record of global temperature since the end of the last ice age," said Candace Major, program director in the National Science Foundation's Division of Ocean Sciences, which co-funded the research with NSF's Division of Atmospheric and Geospace Sciences. "This research shows that we've experienced almost the same range of temperature change since the beginning of the industrial revolution as over the previous 11,000 years of Earth history – but this change happened a lot more quickly."

Taken at face value, the new study does suggest, to paraphrase the old investing caveat, that we should worry about what past performance may have to say about future results.