How do we know the planet is hotter than ever before?

August 12, 2010

Open Knowledge spoke with Stefan Rahmstorf, a climate scientist at the Potsdam Institute for Climate Research, on how it is possible to draw conclusions about Earth's climate patterns over thousands of years.


Allianz Open Knowledge: Temperature measurements go back to the 18th century. How reliable are they?

Stefan Rahmstorf: Some local measurement series in Europe indeed go back that far. But only since the latter half of the 19th century have there been sufficient measurements to determine changes in the globally averaged surface temperature.

The measurements show a global warming of about 0.8 degrees Celsius since the beginning of the 20th Century. Three quarters of this warming has occurred since the 1970s.

The recent gloal warming trend is also confirmed by satellite measurements, which started in 1979. Incidentally, the year from April 2009 to March 2010 was the warmest twelve-month period on record.


How can we know anything about the climate further back in time, during the Middle Ages for example?

For earlier periods we have so-called proxy data. These come from tree rings, ice cores, ocean sediments, corals, or cave stalagmites.

Naturally such data are more uncertain, because the things that can be measured may not only depend on temperature but also on other environmental factors, like rainfall rates.

Therefore one should never rely on just one type of data, but rather see whether different approaches give consistent results.


Some people say that temperatures back then were higher than today. Is that right?

According to these proxy data, the Middle Ages were the warmest era of the past 1,500 years. Only in the last few decades have global temperatures soared above those found in the Middle Ages.

Stefan Rahmstorf, Potsdam Institute for Climate Impact Research: "I must say that from some quarters, the reaction against the idea of human-caused warming is very emotional, resulting in wild conspiracy theories and ad-hominem attacks against climate scientists."
Stefan Rahmstorf, Potsdam Institute for Climate Impact Research: "I must say that from some quarters, the reaction against the idea of human-caused warming is very emotional, resulting in wild conspiracy theories and ad-hominem attacks against climate scientists."

Man-made CO2 can’t have caused warming in the Middle Ages. How do you explain it?

The two main drivers of climate variations in the past one or two millennia were changes in solar activity and volcanic activity. If we drive climate models by these two factors we find similar climate variations in the models as are shown by the proxy data.


Tree rings take us one step further back in time. What do they tell us?

Some tree ring compilations go back thousands of years through much of the current warm period, the Holocene, which has lasted for the past 11,000 years. However, these data are only local.

To get a more complete picture of the Holocene we need to combine data from many sources. In mid- to high northern latitudes the overall pattern is one of cooling.

That is not surprising, since the cycles in the Earth’s orbit caused a much stronger solar radiation and warmer temperatures in the early Holocene in this region.

But these orbital cycles affect different regions in different ways. Much of the tropics has slowly warmed over the Holocene.


Some scientists analyze ice core samples. How valid are these findings?

Ice cores have proven to be an extremely valuable data source. They can be dated quite well. Many parameters can be measured in the ice, even including the composition of the atmosphere from little air bubbles trapped and preserved in the ice.

The Greenland ice cores reach back around 100,000 years. In Antarctica, because of the low snowfall rates, they can go back close to a million years.

The ice cores very clearly reflect the cycles between ice ages and warm periods, driven by the cycles of the Earth’s orbit. But they have also revealed some dramatic regional shifts in climate, which have to do with instabilities in big ice sheets and in the ocean circulation. They have shown that Earth is a sensitive system that can respond in dramatic and surprising ways when it is pushed.


Sediments from ocean floors are one of the oldest sources for climate data. What story do they tell?

The broad picture the sediment data give us is that the climate was much warmer than today during most of the past 400 million years. There are only two periods with large amounts of continental ice: one around 300 million years ago, and the one we are in now since the Antarctic ice sheet started to form about 40 million years ago.


Some people say that CO2 levels some 450 million years ago were way higher than today but that ice sheets were present? Is that true?

I’d say we don’t know this. The data before 400 million years ago are very uncertain—both in terms of dating when there was ice, and in terms of estimating past CO2 levels.

In several instances interpretations of proxy data that suggested physically implausible climate changes turned out to be wrong. Unfortunately, the media get excited about every new piece of evidence that seems to contradict previous knowledge.

In science knowledge grows slowly in little increments, considering many lines of evidence. Very rarely does one piece of the puzzle—fitting or not—overturn the whole picture.


How much of what there is to be known about the Earth’s climate do we know already?

The basic physical principles that rule our climate are known: the Earth’s energy balance and the factors that influence it. We know about changes in the luminosity of the sun or in the Earth’s orbit, in the brightness of the Earth’s surface (albedo) or in greenhouse gas concentrations.

Nothing in the Earth’s history suggests that we are missing a major ingredient. However, there is a huge amount still to be discovered and puzzles yet to be solved. I would recommend climatology to any bright young scientist as an area where exciting discoveries can still be made.

Thilo Kunzemann

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