Climate change is happening; the question is what to do about it. President Obama had one answer: reduce greenhouse-gas emissions aggressively.

Unfortunately, his approach was heavy on cost and light on benefit, and with yesterday’s executive order rolling back those efforts, President Trump has rightly begun the process of reversing it.

But President Trump's own response to climate change appears to be: nothing. That’s not the right answer either.

Most climate policy falls into one of two categories: There is “mitigation,” which means trying to prevent climate change by reducing greenhouse-gas emissions -- think wind farms. And then there is “adaptation,” which means dealing well with whatever change occurs -- think sea walls.

The basic problem with Obama’s mitigation-focused approach is that the overwhelming majority of future emissions will come from the developing world as it grows rapidly. U.S. policy has shown little ability to influence that trend, even when we make brave commitments to incur large costs ourselves.

We can -- and should -- invest in developing new technologies that might reduce emissions more cheaply, but that takes time, and success is not guaranteed.

Without major mitigation, though, adaptation becomes all the more important. And that is a place where good American policy can make a major difference.

For instance, continued research into the likely effects of climate change can inform policymakers and private actors about what to expect, so that they can make better investments.

If certain areas of the country are likely to get wetter or drier, hotter or colder, we should want to know that. Yet the Trump Administration seems inclined -- at least so far -- to dismiss such research and cut its funding as aggressively as possible.

The instinct is understandable, given the research sloppily conducted or misappropriated for activist ends under the previous Administration.

The Obama-era EPA's “Climate Change Impacts and Risk Analysis” project, for example, managed to conclude that the rate of heat-related deaths for New York in the year 2100 would be 50 times higher than it was for Phoenix in 2000 -- even though New York would be nowhere near as hot in 2100 as Phoenix is already.

But that egregious effort does not make good research less valuable. It just underscores the need for research to be well focused and insulated from politics.

Similarly, we should want government planners at every level to take the best existing research into account as they make public investments and set policy that will influence others.

If farmers and resort owners and mayors and naval planners all build with an eye toward how the future might change, then those changes as they arrive won’t be so harmful or expensive.

Yet, in addition to starting the repeal of costly mitigation efforts like Obama’s Clean Power Plan, Trump’s executive order entirely erases an Obama order aimed at “preparing the United States for the impacts of Climate Change.” Many of the points in that program still make sense.

Perhaps the greatest mistake made by those who overinflate the risk of climate change is to forget that our society has a tremendous capacity to adapt and innovate. But it would also be a major mistake to forget that public policy can either foster or hinder that process.

If President Trump dislikes his predecessor’s approach to adaptation he should put forward an alternative. Ignoring the problem entirely is one of the few things he can do that really would make it worse.