How long will the deniers keep denying?
Global temperatures are warmer than at any time in at least 4,000 years, scientists reported Thursday, and over the coming decades are likely to surpass levels not seen on the planet since before the last ice age.
Previous research had extended back roughly 1,500 years, and suggested that the rapid temperature spike of the past century, believed to be a consequence of human activity, exceeded any warming episode during those years. The new work confirms that result while suggesting the modern warming is unique over a longer period.
Even if the temperature increase from human activity that is projected for later this century comes out on the low end of estimates, scientists said, the planet will be at least as warm as it was during the warmest periods of the modern geological era, known as the Holocene, and probably warmer than that.
That epoch began about 12,000 years ago, after changes in incoming sunshine caused vast ice sheets to melt across the Northern Hemisphere. Scientists believe the moderate climate of the Holocene set the stage for the rise of human civilization roughly 8,000 years ago and continues to sustain it by, for example, permitting a high level of food production.
In the new research, scheduled for publication on Friday in the journal Science, Shaun Marcott, an earth scientist at Oregon State University, and his colleagues compiled the most meticulous reconstruction yet of global temperatures over the past 11,300 years, virtually the entire Holocene. They used indicators like the distribution of microscopic, temperature-sensitive ocean creatures to determine past climate.