Hot clues to future warming from Earth’s distant past

We’re in for global warming more severe than most scientists have predicted, according to a January 14 article in the journal Science. EarthSky talked to study author Jeffrey Kiehl, a climate scientist at of the National Center for Atmospheric Research. Dr. Kiehl backs up his claim with climate data from a period on Earth called the late Cenozoic – about 35 million years ago. He said:

Why would I want to go back and calculate temperature for that far back in Earth’s history? The reason for that is that if we continue to burn fossil fuels at the rate we have been burning them for the next 80 or 90 years, then the level of carbon dioxide is going to be very high …

As high as 1,000 parts per million by the year 2100. Kiehl told us the last time we had that much carbon dioxide in the atmosphere was 35 million years ago. That is, during the late Cenozoic – the period he studies. It was a hot, hot world, he said. Global temperatures were about 29 degrees Fahrenheit (16 degrees Celsius) higher than today’s. Carbon dioxide was trapping the sun’s heat – kind of like glass traps heat inside a greenhouse. Kiehl said:

What I did was use a mathematical technique to figure out Earth’s greenhouse effect back in time. By knowing the greenhouse effect 35 million years ago … you can calculate how sensitive earth’s climate system is to increased greenhouse gases such as carbon dioxide.

Dr. Kiehl said many scientists hypothesize the Earth will warm by 3-5 degrees Celsius by the year 2100. But Kiehl has probed the future even further, by looking at climate evidence from Earth’s past – fossil records, geological records, and the like. His new climate analysis of Earth’s late Cenozoic period suggests that, in centuries beyond the year 2100, the world’s average temperatures could increase by a whopping 16 degrees Celsius. For those on the Fahrenheit system, that represents an increase of nearly 30 degrees. He said:

There’s another important aspect to this, and that is, why is there this difference between the calculation that I’ve done based on Earth’s history, and the estimates that people get from climate models today. And there’s a good answer to that. On the longer time scale you have things changing in Earth’s system that aren’t accounted for in climate models today.

Some of those things, he said, are melting of ice over continental landmasses in places like Greenland. He also mentioned the release of methane into the atmosphere by the oceans. Methane is another greenhouse gas. He added:

These processes take a long time. On the time scale of centuries.

EarthSky asked Dr. Kiehl, “Well, doesn’t this mean global warming is just a natural phenomenon? Why should we worry about it if this is just what the Earth does – gets warmer and cooler?” He gave a great answer.

Well, that’s true, but there’s one catch, which as extremely important one, which is the rate at which it happened in Earth’s past, and the rate at which it’s occurring into the future. Which is that if you look at Earth’s past … the carbon dioxide in the atmosphere increased and decreased on very very long time scales. It took us roughly 30 million odd years to go from that high level [of the Cenozoic] to a lower level – what we’re living in now.

But we’re pressing down on the gas pedal of climate, so to speak. Kiehl said that, with our burning of fuel, we humans found an extremely efficient way of taking carbon out of the earth, burning it and putting it back into the atmosphere.

[We’re putting CO2 into the atmosphere] at a rate that is far, far faster than anything the natural carbon processes that have worked in Earth. So in the matter of a century – not 30 million years, but a matter of 100 years – we’re putting all that carbon dioxide back in the atmosphere and warming in the planet.

To offer some numbers: our carbon dioxide level is at about 390 million parts per million right now. We’re going to take it, through our activity – if we continue on course – to 1,000 parts per million within the next 100 years. In the natural world – without human activity or interference – that same change would take eons. Keil told us:

The biosphere, species, vegetation, these organisms are forced to respond to a rate of change that is really phenomenal, in terms of Earth’s history

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