responses over time to take advantage of new knowledge. Iterative risk management strategies must be durable enough to promote sustained progress and long-term investments, yet sufficiently flexible to take advantage of improvements in knowledge, tools, and technologies over time.

In the context of an iterative risk management framework, and building on the analyses in the four ACC panel reports, the committee recommends the following priority actions for an effective and comprehensive national response to climate change:

Substantially reduce greenhouse gas emissions. In the committee’s judgment there are many reasons why it is imprudent to delay actions that at least begin the process of substantially reducing emissions. For instance:

  • The faster emissions are reduced, the lower the risks posed by climate change. Delays in reducing emissions could commit the planet to a wide range of adverse impacts, especially if the sensitivity of the climate to greenhouse gases is on the higher end of the estimated range.
  • Waiting for unacceptable impacts to occur before taking action is imprudent because the effects of greenhouse gas emissions do not fully manifest themselves for decades and, once manifested, many of these changes will persist for hundreds or even thousands of years.
  • The sooner that serious efforts to reduce greenhouse gas emissions proceed, the less pressure there will be to make steeper (and thus likely more expensive) emission reductions later.
  • The United States and the rest of the world are currently making major investments in new energy infrastructure that will largely determine the trajectory of emissions for decades to come. Getting the relevant incentives and policies in place as soon as possible will provide crucial guidance for these investment decisions.
  • In the committee’s judgment, the risks associated with doing business as usual are a much greater concern than the risks associated with engaging in strong response efforts. This is because many aspects of an “overly ambitious” policy response could be reversed if needed, through subsequent policy change; whereas adverse changes in the climate system are much more difficult (indeed, on the timescale of our lifetimes, may be impossible) to “undo.”

RECOMMENDATION 1: In order to minimize the risks of climate change and its adverse impacts, the nation should reduce greenhouse gas emissions substantially over the coming decades. The exact magnitude and speed of emissions reduction depends on societal judgments about how much risk is acceptable. However, given the inertia



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