Over the past several decades, the scientific community has arrived at a consensus that the earth’s climate is being changed by human influences, most importantly the release of carbon dioxide (CO₂) and other “greenhouse gases” (GHGs) into the atmosphere. The most recent estimates by the Intergovernmental Panel on Climate Change (IPCC) indicate that, under a “business as usual” scenario, the average global temperature will rise 2.5 to 10.4 degrees Fahrenheit by the end of the 21st century.¹” This is a significant change: the high end of this range is equal to the change in the average global temperature associated with the end of the planet’s last ice age, 10,000 years ago. But, during that ice age, it took thousands of years to reach this level of warming — not just one century.

The virtual certainty that human influences are causing these significant changes in our climate naturally leads to the questions of what actions to take and when to take them. A previous Pew Center domestic policy brief, entitled The U.S. Domestic Response to Climate Change: Key Elements of a Prospective Program, evaluates possible policy approaches.

This “In Brief” addresses the timing of action to reduce GHG emissions. In October 2001, the Pew Center on Global Climate Change held a workshop inviting leading scientists, economists, and other analysts to discuss this question.² The Workshop on the Timing of Climate Change Policies revealed a consensus that action to address global climate change must begin now if it is to be effective. An immediate signal that initiates action is required in order to provide a smooth and cost-effective transition to a stable concentration of GHGs in the atmosphere — a challenge that will take decades, if not generations, to meet. Workshop participants identified many compelling reasons to begin taking action now, including:

• The reality that current atmospheric concentrations of CO₂ have not been exceeded during the past 420,000 years (the period for which ice core data are available) and will soon exceed a doubling of pre-industrial levels resulting in a situation unprecedented in human history with unknown consequences;
• The potential for catastrophes that defy the assumption that damages resulting from climate change will be incremental, smooth, and linear;
• The risk of irreversible environmental impacts (as compared to the lesser risk of unnecessary investment in GHG reduction or mitigation);
• The need to learn about the pace at which society can begin a transition to a climate-stable economy;
• The likelihood of imposing unconscionable burdens and impossible tasks on future generations;
• The need to create incentives to accelerate technological development that will allow us to address the climate change problem; and
• The ready availability of “no regrets” policies that have very low or even no costs to the economy.