On May 9, 2013, the National Oceanic and Atmospheric Administration in the US recorded CO2 levels in the atmosphere at of 400 parts per million. This signifies a return to the atmospheric conditions similar to those of the Pliocene, which ended about 2.6 million years ago.
The tropical Pliocene
Global Pliocene temperatures were on average about 2–4°C warmer than pre-industrial temperatures. Those temperatures drove an intense hydrological cycle with extreme evaporation and precipitation. It led to extensive rain forests, lush savannas (now occupied by deserts), small ice caps (about two-thirds of the present) and sea levels about 25 meters higher than at present.
Life abounded during the Pliocene. But such conditions mean agriculture would hardly be possible. The tropical Pliocene had intense alternating downpours and heat waves. Regular river flow and temperate Mediterranean-type climates which allow extensive farming could hardly exist under those conditions.
After the Pliocene, the earth’s climate shifted gradually into the Pleistocene. During the following 2 million years, glacial-interglacial periods required species to adapt to rapid climate shifts. These shifts included abrupt warming events actually within glacial periods, where regional warming by several degrees occurred over periods as short as several decades to a century.
Basic physics and chemistry, as well as the geological record, tell us that greenhouse gases are the key factor determining the current climate trend. Current greenhouse gas rise rates exceed those of the Pleistocene cycles by more than an order of magnitude. These rates, which during 2012-2013 reached 2.89 ppm CO2 per year, exceeding any recorded for the last 65 million years, would hardly allow species to adapt to changing climate conditions (see Figure 1).
Will this mean mass extinction?
The current CO2 ppm per year rise rate surpasses CO2 and temperature rise rates during mass extinctions about 55 million years ago and 65 million years ago.
Fifty-five million years ago, large-scale release of methane drove atmospheric CO2 to near-1800 ppm and temperature rise to about 5°C over a period of 10,000 years. (That’s 0.18 ppm CO2year and 0.0005°C/year.)
Sixty-five million years ago, the K-T asteroid impact resulted in a rise of more than 2000ppm CO2 and about 7.5°C over a period of about 10,000 years (or about 0.2 ppm/year and 0.00075°C per year). About 4500 billion tons of carbon was released from impacted carbonates and shale, from ignited bushfires and from ocean warming. The CO2 rise rate was an order of magnitude lower than current rate of 3ppm/year.
What will the world look like?
If we burned all the earth’s known fossil fuel reserves it would lead to atmospheric CO2 levels of around 800 to 1000ppm, high or total melting of the polar ice caps, sea level rise on the scale of tens of meters and disruption of the biosphere on a scale analogous to recorded mass extinctions.
At the same time as CO2 emissions, sulphur dioxide (SO2) is being released, mainly from coal burning. This sulphur is increasing the reflection of the atmosphere and thus regulates changes in temperature, as shown in Figure 2 for the periods following 1950, 1975 and 2001. The trend of rising temperatures slowed in 1950 and 2001 when sulphur emissions increased. Likewise, when clean air policies were introduced in 1975, slowing sulphur emissions, a fast-rising temperature trend resumed. The current rise in coal burning and sulphur emissions are locking the world into a Catch-22 cycle.
Carbon emissions may be self-limiting. It is likely that, before atmospheric CO2 reaches 500ppm, extreme weather events would disrupt industrial and transport fossil fuel-combusting systems enough to lead to reduction of emissions. However, the feedback processes like methane release, forest bushfires and warming oceans will drive CO2 levels further.
When will we act?
The land, oceans and biosphere are now in extreme danger, but it doesn’t seem to be driving the global community to the urgent measures required for a meaningful attempt to arrest the current trend. With few exceptions, the accelerating rate of atmospheric CO2 hardly rates a mention on the pages of the global media, preoccupied as it is with short-term economic forecast, daily exchange rates, share market fluctuations and sports results.
In Australia the language has changed from “the greatest moral issue of our generation” to controversy over a “carbon tax”, diverting the public attention from the climate to a hip-pocket nerve. While we debate the ways to bring about a meaningless 5% reduction in local emissions, we simultaneously develop infrastructure to export hundreds of millions of tons of coal. It all ends up in the same atmosphere.
As Carl Sagan reminded us, on seeing a photograph of Earth taken from Voyager 1 as it left the Solar System
That’s here. That’s home. That’s us. On it everyone you know, everyone you love, everyone you’ve ever heard of, every human being who ever was, lived out their lives … Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity — in all this vastness — there is no hint that help will come from elsewhere to save us from ourselves.
Andrew Glikson isan Earth and paleo-climate scientist at Australian National University. He does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.