From Hockey Stick to Ski Jump
March 8, 2013
In an enormous leap forward, new analyses of temperature records for the last 11000 years have now been published.
Don’t look for the climate denial blogosphere to learn anything from new science. Expect instead a newly focused attack on the facts, – trolls will have to bone up – and a new buzzword.
The older temperature graphs took in only the past 1000 or 2000 years, and were recognizable for the pronounced upward swing at the end. The new, longer graphs include a hump that peaked 9500 years ago, when the planet’s axial tilt was most extreme toward the sun, and have declined slowy until the past century, where we see a dramatic spike.
Less like a Hockey stick, said one scientist. More like a Ski Jump. With a brick wall at the end.
A new study looking at 11,000 years of climate temperatures shows the world in the middle of a dramatic U-turn, lurching from near-record cooling to a heat spike.
Research released Thursday in the journal Science uses fossils of tiny marine organisms to reconstruct global temperatures back to the end of the last ice age. It shows how the globe for several thousands of years was cooling until an unprecedented reversal in the 20th century.
Scientists say it is further evidence that modern-day global warming isn’t natural, but the result of rising carbon dioxide emissions that have rapidly grown since the Industrial Revolution began roughly 250 years ago.
The decade of 1900 to 1910 was one of the coolest in the past 11,300 years — cooler than 95 percent of the other years, the marine fossil data suggest. Yet 100 years later, the decade of 2000 to 2010 was one of the warmest, said study lead author Shaun Marcott of Oregon State University. Global thermometer records only go back to 1880, and those show the last decade was the hottest for this more recent time period.
“In 100 years, we’ve gone from the cold end of the spectrum to the warm end of the spectrum,” Marcott said. “We’ve never seen something this rapid. Even in the ice age the global temperature never changed this quickly.”
“By the year 2100, we will be beyond anything human society has ever experienced,” said study leader Shaun Marcott, a postdoctoral researcher at Oregon State University‘s College of Earth, Ocean and Atmospheric Sciences.
According to NASA, the average global temperature for 2012 was 58.3 degrees Fahrenheit, making it the ninth-hottest year in recorded history. However, there is no single, agreed-upon method of calculating these temperatures, so scientists tend to discuss climate change by highlighting deviations from a specific reference point.
While a 1-degree Fahrenheit increase sounds small, it represents an enormous amount of heat energy. For instance, a 10-degree drop would plunge the world into another period of major glaciation, while every 1.8-degree increase would gradually amount to a roughly 65-foot rise in sea level due to melting polar ice, according to NASA climatologist James Hansen.
Previous efforts to measure past climate conditions have relied heavily on measurements of tree ring thickness. At high latitudes, tree growth is controlled mostly by temperature, so thick rings suggest warm years. But trees don’t live longer than several thousand years, so those efforts have focused on shorter periods of time — just 1,500 to 2,000 years.
Marcott’s graph shows temperatures rising slowly after the ice age, until they peaked 9500 years ago. The total rise over that period was about 0.6 °C. They then held steady until around 5500 years ago, when they began slowly falling again until around 1850. The drop was 0.7 °C, roughly reversing the previous rise.
Then, in the late 19th century, the graph shows temperatures shooting up, driven by humanity’s greenhouse gas emissions.
The rate of warming in the last 150 years is unlike anything that happened in at least 11,000 years, says Michael Mann of the Pennsylvania State University in University Park, who was not involved in Marcott’s study. It was Mann who created the original hockey stick graph, which showed the change in global temperatures over the last 1000 years.
Over the Holocene, temperatures rose and fell less than 1 °C, and they did so over thousands of years, says Marcott. “It took 8000 years to go from warm to cold.” Agriculture, communal life and forms of government all arose during this relatively stable period, he adds. Then in 100 years, global temperatures suddenly shot up again to very close to the previous maximum.
How fast temperatures change is the real issue of climate change, says Mann. “That’s what challenges our adaptive capacity.” Rapid change means farming practices must alter quickly, and preparations for extreme weather events must also be rapidly put in place.
The gradual changes through the Holocene were driven by changes in Earth’s rotation, says Marcott. The planet is tilted about 23° relative to the plane of its orbit, and this tilt increased early in the Holocene before decreasing again. “It sort of wobbles,” Marcott says. A greater tilt increases the amount of sunlight at the poles during summer, and this keeps the planet warmer.
If humans had not begun warming the planet by releasing greenhouse gases, Earth would eventually return to an ice age. “If we were following the orbital trend we’d still be cooling,” Marcott says.
Marcott’s data suggests that the planet is now nearly – but not quite – as warm as at its warmest point in the last 11,000 years. Some climatologists have suggested that it is already hotter, but it is difficult to say for sure due to uncertainties in the data.
Mann says it should be possible to find out. He points out that, according to Marcott’s data, the tropics are currently hotter than at any point during the Holocene. The poles, on the other hand, appear to have been unusually warm in the past. But Marcott’s data set contains a lot of data from the far north, and Mann thinks it might be slightly exaggerating past warmth.
“It’s possible, given the potential bias, that there is in fact no precedent over the past 11,000 years,” says Mann says.
Even if we are not yet hotter than the entire Holocene, we will soon be. Marcott examined the latest climate models’ temperature predictions for this century, and found that by 2100 they were all outside the Holocene range. “The projections for 2100 will be very clearly outside the entire distribution of the data,” Marcott says.