You Think It's Hot Now? Just Wait.

Figure from Steffen et al.: a global map of potential

tipping cascades. The individual tipping elements are

color-coded according to estimated thresholds in global

average surface temperature (tipping points).

It's getting hotter all over the planet.

This week the temperature in Bar Harbor, Maine, reached 91° F (32.8° C). In my 20 years vacationing here, this is easily the hottest weather I've ever experienced.

Up and down the U.S. east coast, cities are sweltering, and temperatures out west are even hotter, with California seeing all-time high temperatures, including the hottest July on record in some areas, which has fed damaging fires across the state. Death Valley is always hot, but this week has been crazy, with temperatures on August 7 reaching 122° F (50° C).

At the same time, Europe is baking under a "heat dome" that has brought unprecedented high temperatures, including 45° C (113° F.) in Portugal. It's so hot that people aren't even going to the beach.

Global warming is here, folks. I know we're supposed to call it "climate change," because it's much more complex than simply warming, but warming is one of the most obvious consequences.

And yes, a single heat wave doesn't prove anything, and weather is not the same as climate. I know. But a just-released study from Oxford University found that climate change made this summer's heat wave in Europe twice as likely.

And now, a new study published last week in the Proceedings of the National Academy of Sciences, says it could get much, much hotter if we don't do something about it. In this paper, an international team of climate scientists led by Will Steffen and Hans Joachim Schellnhuber explain that, thanks to human activities, the planet is well on its way to a "Hothouse Earth" scenario.

In a Hothouse Earth, global average temperatures would rise 4–5° C (7–9° F) and sea levels will rise 10–60 meters (33–200 feet) above today's levels. This would be catastrophic for many aspects of modern civilization. Many agricultural regions would become too hot and arid to sustain crops, making it impossible to feed large swaths of humanity. Low-lying coastal areas would disappear or become uninhabitable without massive engineering efforts, displacing hundreds of millions of people. As Steffen et al. put it:

"The impacts of a Hothouse Earth pathway on human societies would likely be massive, sometimes abrupt, and undoubtedly disruptive."

That's putting it mildly.

One reason this scenario is happening, as the study explains, is that we are very close to "tipping points" beyond which certain changes cannot be stopped. (We may have already passed some of them.) These include losing the Arctic ice cap in the summer, and losing the Greenland ice sheet permanently: because they are basically white, these massive expanses of ice serve as giant reflectors to send much of the sun's heat back into space. Without the ice, the darker planet surface absorbs far more heat, creating a positive feedback effect. Another example is the melting of the permafrost, land that has been frozen for thousands of years and that contains a great deal of carbon in the form of methane. Once that methane is released, it will create further warming.

We are also likely to lose the Amazon rainforest, all of our coral reefs, and huge swaths of boreal forests. (See here for a global map of these tipping points.)

If this seems grim, Steffen and colleagues point out that we still have time to avoid it. They propose that societies must act collectively to create a "Stabilized Earth" at no more than 2° C above pre-industrial levels, which is possible but not easy:

"Stabilized Earth will require deep cuts in greenhouse gas emissions, protection and enhancement of biosphere carbon sinks, efforts to remove CO2 from the atmosphere, possibly solar radiation management, and adaptation to unavoidable impacts of the warming already occurring."

None of this is beyond our abilities. We know what we need to do, but it requires large-scale, coordinated action that many governments must agree on if it's to have an impact. Unfortunately, humans (and our governments) tend to do nothing until faced with an emergency, and the tipping points leading to a Hothouse Earth may not look like emergencies, not at first. For example, Arctic sea ice has been declining steadily for 25 years or more, but because few people are aware of this (and even fewer experience it first hand), it doesn't seem urgent. Yet it is.

So perhaps this summer's heat wave can serve as a wake-up call that we need to pay more attention to our planet's health. Otherwise it's going to get a lot hotter.