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What hockey stick graphs tell us about recent climate change

  • 08 Mar 2013, 15:30
  • Roz Pidcock

Scientists have shown for the first time that global temperature has risen faster in the past century than since the end of the last ice age 11,300 years ago. What's more, by the end of the century, temperatures are expected to surpass the warmest temperatures during that time - making it the hottest period for more than 11 millenia.

A new paper, published today in the journal Science, is the first to reconstruct earth's temperature over the past 11,300 years - the period since the end of the last ice age, known as the holocene.

The data show the speed of global temperature rise in the last century - due largely to human activity - is unprecedented in the holocene.

The new study extends the iconic "hockey stick" graph, so named because it showed relatively little temperature rise from AD 1000 to 1900 - and then a very sharp rise in the 20th century when fossil fuel burning started to take effect.

But that analysis only looked at global temperature for the past 2,000 years. An extra 10,000 years worth of data in the new study allows scientists to look deeper into earth's history. And as climate scientist Michael Mann, creator of the original "hockey stick" graph told Carbon Brief today:

"This new study extends and strengthens that result even further."

Or as lead author Shaun Marcott from Oregon State University in the US explains in the press release:

"We already knew that on a global scale, earth is warmer today than it was over much of the past 2,000 years. Now we know that it is warmer than most of the past 11,300 years."

Rapid warming

In the new study, the team of scientists collected temperature data from 73 locations all over the world. Thermometers have only been measuring temperature directly since about the late 17th century. So to find out about temperatures before then, the scientists looked at a number of natural recorders of past temperature changes, such as fossils buried in seafloor sediments, corals, fossilised pollen on land, ice cores and tree rings. These are known as climate proxies.

Their analysis shows global temperature rose by about 0.6 degrees Celsius over the first half of the holocene, peaking about 7,000 years ago. Current global temperatures haven't quite surpassed that yet, but they're close. All of the Intergovernmental Panel on Climate Change (IPCC)'s scenarios based on how greenhouse gas emissions could evolve show global temperature exceeding the warmest period of the holocene as soon as 2100.

Marcott Et Al ., (2013)
Source: Markott et al., (2013)

The researchers worked out that it was warmer between 2000 and 2009 than it has been over at least 72 per cent of the holocene. And by the end of the century that figure will be 100 per cent, according to IPCC projections.

But it's the speed of temperature change that's most startling, climate scientist Michael Mann says on Andy Revkin's Dot Earth blog:

"The real issue, from a climate change impacts point of view, is the rate of change -because that's what challenges our adaptive capacity. And this paper suggests that the current rate has no precedent as far back as we can go with any confidence."

Masking natural cooling

After the temperature peaked in the mid-holocene, the new data show the earth embarked on a cooling trend because of changes in the position of the earth relative to the sun. Global temperature declined by about 0.7 degrees Celsius over the last 5,000 years, culminating about 200 years ago in what's become known as the Little Ice Age.

The scientists say the earth should be experiencing the lowest point of that temperature decline now, but much faster temperature rise this century due to human-caused emissions has dominated instead. Marcott explained in the press release:

"As the earth's orientation changed, Northern Hemisphere summers became cooler, and we should now be at the bottom of this long-term cooling trend - but obviously, we're not."
The authors explain a bit more in the paper:

The rate of temperature rise due to human activity means "global temperature … 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 approximately 5,000 before the present."

In the UK,  The Times The Independent, the  Daily Mail and last night's  Evening Standard picked up the story today. All four headlines, however, incorrectly say the earth is the hottest it's been in 11,300 years. To reiterate the study's conclusions, it's the rate of temperature rise that's unprecedented since the end of the last ice age, not the current temperature. At least not yet anyway.

Criticism

Only just published today, the new historical temperature record already has its critics. Robert Rohde, chief scientist with the Berkeley Earth Surface Temperature ( BEST) study has suggested the average time gap of 160 years between data points in the new study is enough to hide rapid changes in temperature - like the temperature rise experienced last century.

Mann told us he was initially concerned the spacing of the data points "might be an issue". But, he added:

"I'm less concerned now that I have read the paper over more carefully … I'm relatively convinced that they have the resolution to capture a century-long warming trend in the past were there one comparable to the recent trend."

What's more, the authors explain in the paper that it's this type of relatively sparse data record, compared to tree rings for example, that allows them to extend the data set so far back in time.

The Global Warming Policy Foundation's David Whitehouse has also tried to play down recent warming by suggesting "temperatures seen in the 20th century were about average for the holocene". But that glosses over the fact that experiencing the same range in temperature as seen over the whole of the holocene in the space of a century means it appears as a huge spike in the historical record.

Critical timing

With the next IPCC assessment on the science of climate change due later this year, the new and improved glimpse into earth's distant past this paper offers is just in time to be included. As a step forward in understanding earth's past temperature record, it should help put the scale of recent climate change into a much wider context.  

 

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