Who’s Most Responsible for Global Warming?

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This week, we’re previewing Scott Pruitt’s testimony, answering your climate questions and updating you on current events. (That last one is a bad pun — you’ll see why.)

Scott Pruitt on Capitol Hill

It might not rise to the level of James Comey’s testimony before the Senate last year, or even Mark Zuckerberg’s appearance on Capitol Hill this month, but when Scott Pruitt, the Environmental Protection Agency administrator, appears before two congressional committees on Thursday it is certain to be worth watching.

Environmental groups have already planned morning protests ahead of Mr. Pruitt’s testimony, while Senate Democrats intend to hold a news conference demanding “ethics accountability” from the administrator.

Once inside the room Mr. Pruitt can expect to be hit with questions from both parties on a long list of ethics issues, most of which are now targets of 10 different federal inquiries. Here’s our guide to the investigations into Mr. Pruitt’s practices. Some of the biggies: A $50-a-night condo rental from the wife of an energy lobbyist; frequent first-class travel; a 20-person, round-the-clock security detail; and the purchase and installation of a $43,000 secure phone booth in Mr. Pruitt’s office.

An internal E.P.A. document reviewed by The New York Times shows Mr. Pruitt is prepared to deny responsibility for the spending or ethics issues. But congressional hearings have been known for surprises. Ken Calvert, the top Republican on the House Appropriations panel that will question Mr. Pruitt on Thursday afternoon, told my colleague Coral Davenport, “We’re going to ask him every question you can imagine.”

You can follow Mr. Pruitt’s testimony on our live briefing beginning at 10 a.m. Eastern time. We’ll include a live stream.

Your burning climate question: Who’s responsible?

We love answering your questions about climate change. Send us yours via the form at the bottom of our climate Q. and A. This one comes from Philip Beere of Los Angeles:

What countries are most responsible for carbon emissions?

Last year, when President Trump announced he would pull the United States out of the Paris Agreement, my former colleague Justin Gillis and I took a look at America’s historical responsibility for climate change.

Using data from the Carbon Dioxide Information Analysis Center going back more than 160 years, we compared how much planet-warming CO2 has been released by every country since the industrial revolution. You can see the results in the chart.

Today’s highly industrialized economies — the United States and Europe — got a big head start on burning fossil fuels. But China and other developing nations have ramped up output in recent years.

In total, the United States pumped more carbon dioxide into the atmosphere than any other nation between 1850 and 2014, the latest year for which the center’s data is available. The European Union, including Britain, was the second-largest source of fossil-fuel emissions over that period; China came in third.

But China is today’s biggest emitter, by a mile.

The rapidly industrializing country overtook the United States as the world’s biggest source of carbon emissions in the mid-2000s, and has doubled its output since then.

In 2014, China released 10.3 billion metric tons of carbon dioxide from fossil fuels and industry; the United States released more than 5.2 billion metric tons that year.

(Carbon emissions from both countries decreased slightly by 2016, according to the latest data from the related Global Carbon Project. But 2017 estimates suggest that Chinese emissions ticked back up last year.)

China’s emissions per person are still much lower than the United States’.

China may emit twice as much carbon dioxide as the United States today, but the country is home to four times as many people (about 1.4 billion compared to 328 million). Divvying up national emissions by population gives us a different view of “responsibility.”

Small countries with fossil-fuel-intensive economies, like Qatar and the United Arab Emirates, tend to top the per capita emissions list. But among more populous nations, the United States, Canada and Australia rank highest.

In 2014, the average American was responsible for more than twice as much carbon dioxide released into the atmosphere (16.2 metric tons per person) as the average Chinese citizen (7.5 metric tons); two and a half times as much as the average Briton (6.5 metric tons); and 10 times as much as the average Indian (1.7 metric tons).

Ups and downs in the Atlantic

The Atlantic Meridional Overturning Circulation, a conveyor belt of ocean currents that move warm, salty surface waters northward and colder, deeper water southward, is in the news again.

You may remember the AMOC from the 2004 Hollywood blockbuster “The Day After Tomorrow.” In that film the AMOC collapses — the currents slow way down — and a rapid deep freeze ensues that makes the typical cold spell look like a day at the beach.

The science, if not the movie, was widely panned. But researchers who study ancient climates know that the currents (which include the Gulf Stream) have collapsed in the past. Because the AMOC serves to transfer heat from the tropics into the North Atlantic, where it warms the atmosphere, when it collapses the Northern Hemisphere can cool relatively rapidly. That in turn can affect patterns of precipitation and atmospheric circulation around the world (but nothing like what was portrayed in the film).

But that’s ancient history, and those collapses occurred when sea levels were much lower than they are today. What’s happening with the AMOC now?

Climate models have suggested that as the world warms the AMOC should weaken somewhat, because warmer water is more buoyant and because more precipitation, coupled with melting of the Greenland ice sheet, should reduce the salinity of the surface waters.

“But nobody really knows if that instability will occur in the future as it did in the past,” said Summer K. Praetorius, a paleoclimatologist with the United States Geological Survey in Menlo Park, Calif.

Direct measurements of current flow taken since the mid-2000s do, in fact, show some weakening. (And because that has led to some cooling, “it’s not all bad,” Dr. Praetorius said. “It’s probably been dampening global warming over the last few decades in some sense.”)

But is the weakening a result of climate change, or is it a reflection of natural variability unaffected by warming?

Two papers published in Nature this month don’t fully answer that question, but they provide some context. One suggests that the weakening actually began about 150 years ago. The other suggests the weakening began more recently, in the middle of the 20th century.

The second study supports the idea that climate change is causing the weakening, because by the middle of the 20th century warming was beginning to take hold. But the first study suggests otherwise, said Dr. Praetorius, who wrote a perspective article about the papers, also in Nature.

What about the future? An abrupt collapse of the AMOC this century could result in several feet of additional sea level rise and changes in hurricane activity, among other effects. “If it would completely collapse in 10 years you would see an enormous signal that would completely overpower at least for 20 years anything related to global warming,” said Sybren Drijfhout, a professor of climate dynamics at Utrecht University in the Netherlands.

While the models generally agree that there will be a weakening of the AMOC, they also don’t expect a complete collapse. “It definitely doesn’t say yes, we’re on the verge of an imminent collapse,” Dr. Praetorius said. “Most people don’t think that’s an option anytime soon.”

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