Currently, the Earth is a hot item, breaking records of 120,000 years, according to a new study. It might even go higher, overtaking a record of more than 2 million years. Due to the greenhouse gas emissions since the Industrial Revolution, the Earth is probably committed to a five-degree Celsius hike for the next few millennia.
Carolyn Snyder, now a climate policy official at the U.S. Environmental Protection Agency, drew out a 2-million-year temperature record, which worked out to be longer than an earlier 22,000-year report. She made a temperature reconstruction as part of her doctoral dissertation at Stanford University that was published in Nature.
She did not take records of temperature for a single year, but examined and took averages of 5,000 years. For her dissertation, she went back by a couple of million years. Snyder reconstructed her record based on 61 "sea surface temperature proxies" all over the world. They included ratios between magnesium and calcium, species makeup and acidity.
However, she explained that going back further in time, especially after half a million years makes the proxies less available. Hence the estimates are not so definite.
She agreed that her measurements are "rough estimates with large margins of errors." Still, the temperature changes corresponded well to carbon dioxide levels.
The most recent 5,000 years, especially the last 125 years of industrial emissions tend to be heat-trapping gases. They work out to be warmer than 120,000 years ago or so. The two interglacial time periods that she tracked included 120,000 years ago and another nearing 2 million years ago. Both years, which were the warmest, were 3.6 degrees warmer than that recorded through the current 5,000-year average.
Looking into the future, Snyder calculated that if climate factors equal those in the past - if that happens - Earth is "already committed to another 7 degrees or so of warming" for another thousand years.
"This is based on what happened in the past," Snyder said. "In the past it wasn't humans messing with the atmosphere." The causes of carbon dioxide levels changing in the past included some slight shifts in Earth's orbital tilt.
A number of scientists have appreciated Snyder's work, even though past estimates may not be so easy to track. Jeremy Shakun of Boston College said "Snyder's work is a great contribution and future work should build on it." But he still felt that her future warming estimates are a bit "unrealistic" and does not seem to match historical time periods of similar carbon dioxide levels .
On the other hand, there are many critics of her work. "This is simply wrong," said Gavin Schmidt, director of NASA's Goddard Institute for Space Studies. "The actual committed warming is only 0.5 to perhaps 1 [degree Celsius] - and nothing in the study changes that. You have this chicken and egg situation, where ice changes, which causes CO2 to change, which causes ice to change, and so on and so forth."
"This research cannot and does not provide a forecast or prediction for future climate change," he added. "All we can say is, if we take the past relationship [between temperature and CO2] and translate it forward, this is what we get."
