How do scientists know what they know?

Amid the horror and uncertainty of a global health crisis it can be easy to forget that another worldwide disaster is unfolding, although much more slowly.

Global warming is happening, and its effects are being felt around the world. The only real debates are over how fast and how far the climate will change, and what society should do — the global-warming equivalents of lockdowns and social distancing — to slow or stop it and limit the damage.

As of now, the damage seems to be getting worse. As I wrote in December, impacts that scientists predicted years ago — including severe storms, heat waves and the melting of glaciers and ice sheets — are accelerating.

The coronavirus pandemic can seem overwhelming because of its sheer scope; so can climate change. As a science writer at The Times for more than 20 years, I’ve learned that, to avoid being overwhelmed, it helps to start by understanding one part of the larger problem.

So let’s take a closer look at one piece: what’s happening at the top of the world, the Arctic. It’s a good place to understand the science of climate change, and, it turns out, a critically important one to understand its effects.

Since the mid-1990s, the Arctic has been warming faster than any other region of the planet: currently, at least two and a half times as fast. (Last year, average air temperatures were about 3.5 degrees Fahrenheit, or 1.9 degrees Celsius, higher than the average from 1981-2010.)

In large part, the Arctic is warming the way the rest of the world warms, only up north the process has run amok.

As the concentration of carbon dioxide and other greenhouse gases increase in the atmosphere, so does the amount of heat they trap. But the source of that heat is sunlight striking the Earth, and the amount of heat radiated differs depending on the surface the sunlight hits. Just as a black car gets much hotter than a white car on a sunny day, darker parts of the planet absorb more sunlight, and in turn radiate more heat, than lighter parts.

The Central Arctic is all ocean — dark water that is covered, to a varying extent, by light ice. The ice absorbs only about 30 to 40 percent of the sunlight hitting it; the rest is reflected. Ocean, on the other hand, absorbs more than 90 percent.

As the Arctic warms more of the ice disappears, leaving more dark ocean to absorb more sunlight and radiate even more heat, causing even more loss of ice. It’s a vicious cycle that contributes to rapid warming in the region.

Is this happening at the South Pole as well? No, because while the Arctic is mostly water surrounded by land, Antarctica is the opposite, a huge land mass surrounded by ocean. Some of the ice that covers the continent is melting, but no dark ocean is being exposed. (That’s not to say that the continent isn’t losing ice: it is, mostly through calving of icebergs and melting of the undersides of ice shelves.)

In the Arctic, currents and winds flow out of the region and affect weather elsewhere.

Weakening of the high-altitude winds known as the polar jet stream can bring extra-frigid winter weather to North America and Europe. Cold snaps like these have occurred for a long time although, because of global warming, studies have found that they are not as cold as they used to be. But some scientists now say they think Arctic warming is causing the jet stream to wobble in ways that lead to more extreme weather year round, by creating zones of high-pressure air that can cause weather systems — the ones that bring extreme heat, for example — to stall.

Arctic warming may also be affecting climate over the longer term. As Greenland’s ice sheet melts, the fresh water it releases lowers the saltiness of the nearby ocean. These salinity changes may eventually have an effect on some of the large ocean currents that help determine long-term climate trends in parts of the world.

As climate researchers are fond of saying, what happens in the Arctic doesn’t stay in the Arctic.

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