Over the past two weeks, hourly CO2 averages measured at the Mauna Loa Observatory have exceeded 400 parts per million for the first time. Over the next month, daily averages will likely exceed the 400 ppm milestone. By 2014, one or two months will show an excess of 400 ppm CO2 and by 2016 yearly averages will likely reach or exceed that extraordinarily high level.

To find comparable CO2 levels in Earth’s geological past, one has to venture back in time 2.2 to 4.5 million years to the Pliocene climate epoc. So long ago, humankind, as we know it, was merely a glimmer in Earth’s eye. And the world was filled with strange animals and plants, many of whom do not survive today. The grasses and grains which would become the basis for world agriculture were just beginning to emerge. But they were not as plentiful nor as prosperous as they are today.

During that time, sea levels averaged 75 feet higher than today, temperatures averaged 3-4 degrees Celsius warmer globally and 8-10 degrees hotter at the poles. Ellesmere Island, covered by glaciers today, hosted a forest. If CO2 remains above 400 ppm for any significant period, we can expect an eventual return to these conditions through a chaotic transition of glacial ice melt, major weather changes, major ocean changes, increasing air and ocean temperatures, and other dangerous and disruptive climate feedbacks as the Earth system seeks a new equilibrium. And this begs the question, will those same grasses and grains that developed into such abundance over this time survive and prosper through such a transition? The fate of human civilizations may well hang on the answer to this question.

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In the past 150 years, worldwide CO2 has increased by about 120 parts per million. At the current rate of increase, it will take a little more than two decades to reach 450 parts per million. In the context of geological history, this pace of increase is blindingly fast. Usually, an increase of 10 parts per million CO2 may occur over the course of 1000 years. Even the most rapid increases estimated in the last 60 million years were half as fast as those ongoing today.

To get an idea of the immensity and rapidity of this pace of change, one need only look at the above graph. The graph starts 800,000 years ago. And, as you can see, CO2 concentrations regularly range between 300 ppm during interglacials and 175 ppm during ice ages. This progression continues unabated until we reach the present day when, at the very end of the graph, levels shoot like a rocket toward 400 ppm. In short, humans have pushed the Earth rapidly and radically toward a different climate. And we have yet to fully witness how this extremely radical pace of change will alter our world.

Even worse, the pace of change is increasing. Humans added about 1 ppm CO2 to the atmosphere during the 1950s. We are now adding more than twice that level each and every year. 2011, the most recent year of recorded carbon emissions, showed the highest amounts ever dumped into the atmosphere. This is a pace that can no longer be sustained without serious and severe consequences.

If fossil fuel emissions continue unabated over the next few decades, the changes due for 400+ ppm CO2 will be locked in for millenia. Even worse, if emissions continue to increase as they have and if climate feedbacks such as biosphere carbon and methane release begin to emerge, the world falls very rapidly onto the path of 600-1000 ppm CO2 by or before the end of this century. Following such a path would result in extraordinarily harmful and powerful changes to the Earth’s environment. Such changes would be so powerful and unprecedented that it is doubtful humans could effectively adapt to them.

Some have said that 400 ppm CO2 is an arbitrary number. And that may well be the case. However, it marks a threshold of increasing danger and risk of harm. It is a sign-post showing that we are running out of time to reign in the worst impacts of climate change. It is a clear signal that we need to seriously address, reduce and eliminate greenhouse gas emissions over the course of the next few decades and that we must do so with a resolve not yet seen in the current crop of world leaders.

This is both our challenge and our peril. We simply must respond. We simply must reduce and eliminate carbon emissions. Otherwise we are headed for a very, very dangerous world climate. One which may well be impossible to adapt to.

Links:

The Keeling Curve

Understanding Climate Change is Simple. Want to Stop Temperature Increases? Halt Greenhouse Gas Emissions.