As they contemplate this question, scientists have settled on 3.6°F above average temperatures about a century ago, before they began to rise as a result of the Industrial Revolution. Since the global average has already risen 1.3°F, that means we have only 2.3°F to go before we hit 3.6°F.

Because the problem of climate change is almost certainly anthropogenic, the solution, insofar as there is one, must be the same.

That now-canonical figure, 3.6°, sounds suspiciously precise, especially in Fahrenheit.

It suggests that we can predict within a tenth of a degree when the situation might become catastrophic. On the Celsius or (centigrade) scale, which most of the world uses, the equivalent of 3.6°F is 2°C, which sounds less like a tipping point and more like a focal point on the spectrum of global warming as the disruptive effects grow in frequency, multiplicity, severity and unpredictability.

The hydrological factor in climate change is another example of questions that are still under debate. Two-thirds of the earth’s surface is water. Warming causes evaporation, and vapor traps heat in the atmosphere, which adds to warming. But vapor also adds to cloud cover, which has two effects: It too traps heat at the earth’s surface, but it also reflects heat from the sun back into space, which partially offsets warming.

Scientists are not yet sure about the net effect, although their tentative judgment is that increased cloud cover probably raises temperatures. By contrast, there is little doubt that the accelerated melting of ice caps and glaciers tends to swell rivers and raise sea levels.

As experts combine what they know with what they suspect may happen, they can imagine Biblical-scale floods, droughts and famines. Manhattan and much of Florida under water, breadbaskets turned into wastelands, a major change in the Gulf Stream that could bring Siberian winters to what is now temperate Europe — and the buckling of Arctic permafrost that could release tens of billions of tons of methane, a greenhouse gas that is 20 times more powerful in its heat-trapping effect than carbon dioxide.

These and other “perturbations” — the term scientists use for disturbances that result from changes in climate — would constitute not just an environmental and humanitarian disaster but a geopolitical one, particularly if they interact in ways that are mutually exacerbating.

Defense and intelligence agencies of the U.S. government are concerned about global warming becoming a cause of political instability soon enough in the future to make it a factor in U.S. strategic planning today.

As experts combine what they know with what they suspect may happen, they can imagine Biblical-scale floods, droughts and famines.

Because the problem of climate change is almost certainly anthropogenic, the solution, insofar as there is one, must be the same: a change in human activity that counteracts or, as we have learned to put the goal more modestly, mitigates the consequences of our 200-year accidental experiment.

Scientists and economists believe the world can stay below the 3.6°F/2°C threshold at a reasonable cost. That calculation takes into account the cost if we do not act and if the consequences of global warming are as severe as science indicates they might be. Precisely because there is uncertainty about where on the temperature scale the danger zone is, we should try to stay as much below the 3.6°F threshold as possible.

Moreover, we need to start reductions now in order to slow temperature rise later. Even if we could flip a switch and shut down all emissions, gases that are already in the atmosphere will continue to trap heat for some time to come.

Once emitted into the atmosphere, a molecule of carbon dioxide, or CO2, lingers for decades. So gases emitted today are added to ones that have been around for 50 years or more.

The current concentration of CO2 in the atmosphere is about 385 parts per million (ppm) and growing by two ppm each year. If we continue with current warming trends, the globe could keep warming for millennia. Even if the human species is biologically resilient enough to survive for centuries, the human enterprise may well be hard to maintain in anything like its current form.

Today, humanity is cumulatively emitting, on a yearly basis, around 30 gigatons of CO2. A gigaton is a billion tons. Thirty gigatons is about the weight of 8,000 Empire State Buildings, which, if stacked one on top of another, would reach almost 2,000 miles into space.

Of those 30 gigatons of CO2 that will be emitted this year, just under six gigatons are from the United States. To keep CO2 concentrations below 400 ppm and thereby keep temperature rise below 3.6°F, we should use the next four decades to cut the current output of 30 gigatons a year approximately in half.

Thirty gigatons is about the weight of 8,000 Empire State Buildings, which, if stacked one on top of another, would reach almost 2,000 miles into space.

So that is another target for mitigation: a staged process that would bring the global annual output down to 15 gigatons a year by 2050. To reach that goal, we have to build a new worldwide system for generating and using energy.

We have to begin quickly in order to achieve the bulk of the necessary cuts between 2020 and 2035 so that there is some hope that, by 2050, emissions will have come down to 15 gigatons, concentrations will have stabilized below the 400 ppm level — and temperature rise will have flattened out before hitting the 3.6°F mark.

At the heart of this mammoth undertaking is a transition from a high-carbon to a low-carbon global economy — that is, one that is powered as much as possible by forms of energy that do not burn fossil fuels and therefore do not pump CO2 into the atmosphere.