Climatologists have extended the record of global temperatures
back more than 10,000 years, to the dawn of agriculture, showing
how fast humans are changing the climate.
The iconic "hockey stick" graph is one of the most striking pictures of
how temperatures have changed on Earth throughout history. It uses
a number of "proxies" for
past temperature change, such as data from tree rings, ice cores,
corals and fossilised pollen, to find out how warm the Earth has
been at different points throughout history.
You're probably familiar with what the resulting graph looks
like -- a long flattish line with a dramatic, unprecedented
increase in the last fifty years.
Most recreations of the "hockey stick" graph only extend 2,000
years or so into history -- approximately the limit for data from
tree rings. However, one of the criticisms
of the graph, often from geologists, is that this isn't a
long-enough time period to ascertain whether higher temperatures
are "normal" or not.
So a group of climatologists, led by Shaun Marcott,
reconstructed temperatures much further back -- around 11,000
years, to the very beginning of human agricultural history and the
revolution, when humans stopped being hunter-gatherers.
To get the data, they studied a large number of tiny fossilised
ocean shells. By studying oxygen isotopes in the shells, Marcott
was able to ascertain the temperature of the Earth at the time the
plankton was alive.
"There's been a lot of work that's gone into the calibrations,
so we can be dead certain [the shells] are recording the
temperature we think they're recording," he told Mother Jones.
In the extended timeframe, Marcott found that temperatures in
about 20 percent of this time were higher than they are today.
However, a six-thousand-year cooling trend has been dramatically
halted in the space of a very short timeframe; temperatures have
never changed as fast as they're changing today.
"What we found is that temperatures increased in the last
hundred years as much as they had cooled in the last six or seven
thousand," he said. "In other words, the rate of change is much
greater than anything we've seen in the whole Holocene [the
current geological epoch, which started around 12,000 years
Marcott's research was published