Climate change isn’t just a problem for future generations. The impacts of a warming climate are happening now, with higher temperatures, more extreme droughts, larger forest fires, melting ice caps, more erratic weather and rising sea levels.

Things look grim. But to Hal Harvey, the founder and CEO of Energy Innovation, a San Francisco firm that studies clean energy and other climate policies, the best approach is to view the challenge of climate change as one big math problem. California, the United States and the world can dramatically cut carbon emissions and limit warming over the coming decades by using technologies we already possess, he says. But we don’t have much time. And we need to focus on practical changes that can reduce emissions the fastest, not feel-good trends.

A Stanford-trained engineer, Harvey is the author of “Designing Climate Solutions: A Policy Guide for Low-Carbon Energy.”

He also is former CEO of the Energy Foundation, a philanthropic group that funds clean energy projects in the U.S., India, China and other countries. From 2002 to 2008, he worked as environment program director at the William and Flora Hewlett Foundation.

This conversation has been condensed and edited for clarity and length.

Q: How much of our future climate is already baked in and how much can we still change?

A: We’ve probably baked in something like 1 or 1.5 degrees centigrade (1.8 to 2.7 degrees Fahrenheit). That doesn’t sound like much. But what happens when you change the average temperature is that you change the extremes dramatically. But we’re still at a level where humanity can adapt.

Q: How much time do we have?

A: The United Nations Intergovernmental Panel on Climate Change has argued that we need to drive emissions down radically in the next dozen or 15 years. We need to do most of the right things very promptly to avoid runaway change.

Q: You note in your book that there are more than 200 countries in the world, but just 20 of them produce nearly 80 percent of global emissions.

A: It is 20 countries. If you succeed in those 20 you will be fine. And if you fail in them you won’t be.

There are about a dozen policies that are extremely effective at reducing carbon dioxide emissions, and there are hundreds that I would say are decorative.

Going through the analysis (of possible policies with Energy Innovation’s online calculator, at www.energypolicy.solutions) is very empowering. It informs what you should do on Monday morning as a business leader, as a philanthropist, as a voter, as a citizen of the world.

Q: What do you see as the most effective things that those 20 countries can do in the next 10 or 15 years?

First, transform the power grid. It’s much faster and cheaper than most people realize. There have been fantastic cost reductions in solar, wind, batteries, LED lights and many other technologies. It’s now cheaper in most of the world to build a brand new wind or solar farm than to simply pay the operating costs of an existing coal plant.

Second, electrify everything: Electric vehicles, electric-heated and cooled buildings, and increasingly, electrified industry.

Third, we need to electrify transportation at a very rapid pace. We have these great learning curves for batteries. The more you build, the cheaper they get.

Solar prices have dropped more than 80 percent in the last 10 years. Wind more than 60 percent. Offshore wind, which is an incredibly amazing technology, has dropped by 50 percent in the last four years alone. LEDs have dropped by more than 95 percent in the last five years.

We have technology tailwinds. We have economic tailwinds, and in many states we have political tailwinds. But nationally, we have political headwinds. And everywhere we have status quo headwinds. The world wants clean energy, less asthma, less climate change. But they don’t have as pointed an interest as somebody who just invested $200 million in a natural gas power plant.

Q: Are there any other big strategies that these big 20 countries can use to get the most bang for the buck?

There’s no reason we should have cars get less than 50 mpg. The car companies know how to do it. It’s cost effective for the consumer. You can have a pickup truck that gets 38 mpg or better. The auto companies scream bloody murder, but they did the same thing with seat belts and airbags and safety standards and tailpipes and catalytic converters.

Then there are buildings. An extremely strong building code is magic. If you build a great building it uses very little energy. California has a great building code. We use about 70 percent less energy post-code than we did pre-code. Buildings last a long time, so build them right. This is most important in China and India and Southeast Asia, because that’s where most of the world’s buildings are going up.

Q: It sounds like you’re saying we don’t have to invent all sorts of baroque, complicated new systems to suck carbon out of the atmosphere or anything like that?

A: That’s emphatically, emphatically the case. People who tell you we have to invent a new technology, that’s a recipe for delay. And delay is fatal. The trick is deploy, deploy, deploy.

Q: What about the US? We have a president who denies the science of climate change and who has put lobbyists from the coal and oil industry in office to run the EPA and other agencies.

A: Most energy and climate policy is actually done state-by-state in America. States set utility regulations, they set building codes, they allocate funding for metropolitan planning, and they can set tailpipe standards for vehicles and appliance standards.

In many ways we are doing OK. But that said, the Trump administration, for reasons I cannot fathom, has been incredibly destructive. Tearing up the Paris Treaty was a very bad idea. Trying to reverse EPA standards one after the other is an incredibly bad idea. It’s almost as if they want to cause asthmatic children or accelerated rates of cancer. It’s just wantonly destructive.

Q: What about the folks who deny the science?

A: Science doesn’t care whether you believe in it or not. If you stop believing in gravity you won’t float away. You can waste a lot of breath trying to convince somebody who’s fundamentally not interested in science that scientific results matter.

Two-thirds of all renewables in America have been put in red states. They didn’t do it because they had an Al Gore-style epiphany. They did it because they wanted to save money. That’s fine with me. Get on whatever horse is riding in your direction.

Q: What actions can individuals take that make the most difference?

Get your house well insulated. Buy an electric car or a super-efficient car. That’s most of it. And get out of the car and on the bike whenever you can. Then after you put solar panels on your roof and insulate your house, invite your neighbors over. People are more informed by their peer group than by any expert.

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Hal Harvey

Age: 58

Position: Founder and CEO of Energy Innovation, a San Francisco firm that analyzes climate and energy policy.

Hometown: Raised on a cattle ranch in Snowmass, Colorado.

Residence: Marin County.

Education: B.S. and M.S. degrees from Stanford University in engineering, specializing in energy planning.

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Five facts about Hal Harvey

– Harvey designed built solar homes as an early job.

– He built his own electric car and powered it with solar panels in 1992.

– He rides a Stromer electric bike to work over the Golden Gate Bridge every day.

– Harvey enjoys all kinds of live music, from acoustic rock to big band.

– He has served on energy panels appointed by Presidents George H.W. Bush Sr. and Bill Clinton.