Beyond a Doubling

The Timing of Climate Change Policy

Beyond a “Doubling”

When carbon dioxide or other GHGs are emitted into the atmosphere, they remain there for a period ranging from years to centuries, and in some cases even millennia, before being removed through natural processes. Increasingly over the past century, human activities have resulted in the release of GHGs at rates faster than they can be removed. The resulting accumulation of GHGs in the atmosphere determines, in large part, the severity of changes in the earth’s climate.

The average concentration of CO₂ in the atmosphere during pre-industrial times was about 270 parts per million (ppm), and it is about 370 ppm today. Most analyses of the damages that might result from climate change assume that atmospheric concentrations of CO₂ will be twice pre-industrial levels. Under the IPCC’s 2001 “business as usual” emissions scenario, continuing on our current emissions path will produce concentrations of 550 ppm by 2060. However, given current emissions trends, it will be extraordinarily difficult to stabilize CO₂ concentrations at a mere doubling. Emissions would have to decline 60 to 80 percent by 2100 — and potentially decrease further in the future — in order to stabilize CO₂ concentrations at twice pre-industrial levels.

Thus, even if immediate steps are taken to limit emissions, the atmosphere’s CO₂ concentration could move beyond a “doubling” to as much as a “tripling” by the end of the 21st century. This buildup will occur not only because of steady increases in CO₂ and other GHG emissions, but because the planet’s most effective mechanisms for absorbing CO₂ (i.e., through carbon “sinks” that absorb and store CO₂, such as the upper ocean or forests) will become saturated.

A tripling of global atmospheric CO₂ concentrations is likely to have much more severe consequences for climate than those estimated for a doubling — from temperature increases to changes in patterns of severe weather to a centuries-long rise in global sea level. In fact, climate scientists are only beginning to consider the magnitude of impacts associated with CO₂ concentrations beyond a doubling, and remain limited in their abilities to predict the consequences with confidence.³ 

Thus, perhaps the most pressing reason to begin acting now to address climate change is that, in simplest terms, we are entering unknown climate territory. We are in danger of irreversibly causing climate change that we are only beginning to understand and that is far beyond what we have experienced to date.

NEXT: The Prospect of Catastrophe

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