Why a half-degree temperature rise is a big deal
By Bob Silberg, NASA’s Jet Propulsion Laboratory
Image credits from left to right: Dave/Flickr Creative
Commons/CC BY 2.0; Acropora at English Wikipedia; and Martin
Haas/Shutterstock.com.
The Paris Agreement, which delegates from 196 countries
hammered out in December 2015, calls for holding the ongoing rise in global
average temperature to “well below 2 °C above pre-industrial levels,” while
“pursuing efforts to limit the temperature increase to 1.5 °C.” How much
difference could that half-degree of wiggle room (or 0.9 degree on the
Fahrenheit scale) possibly make in the real world? Quite a bit, it appears.
The European Geosciences Union published a study in April
2016 that examined the impact of a 1.5 degree Celsius vs. a 2.0 C
temperature increase by the end of the century, given what we know so far about
how climate works. It found that the jump from 1.5 to 2 degrees—a third more of
an increase—raises the impact by about that same fraction, very roughly, on
most of the phenomena the study covered. Heat waves would last around a third
longer, rain storms would be about a third more intense, the increase in sea
level would be approximately that much higher and the percentage of tropical
coral reefs at risk of severe degradation would be roughly that much greater.
But in some cases, that extra increase in temperature makes
things much more dire. At 1.5 C, the study found that tropical coral reefs
stand a chance of adapting and reversing a portion of their die-off in the last
half of the century. But at 2 C, the chance of recovery vanishes. Tropical
corals are virtually wiped out by the year 2100.
With a 1.5 C rise in temperature, the Mediterranean area is
forecast to have about 9 percent less fresh water available. At 2 C, that water
deficit nearly doubles. So does the decrease in wheat and maize harvest in the
tropics.
On a global scale, production of wheat and soy is forecast
to increase with a 1.5 C temperature rise, partly because warming is favorable
for farming in higher latitudes and partly because the added carbon dioxide in
the atmosphere, which is largely responsible for the temperature increase, is
thought to have a fertilization effect. But at 2 C, that advantage plummets by
700 percent for soy and disappears entirely for wheat.
Three climate scientists at NASA’s Jet Propulsion
Laboratory, who were not involved with this study, shed some light on the
study’s results, starting with the impact on agriculture.
Corn plants with no corn
Why does a half degree of temperature increase make such a
difference to some of the crops that were studied? For one thing, a half degree
averaged out over the whole world can mean much more of an increase in some
locations and at certain times.
“Most of that temperature change may occur during a small
fraction of the year, when it actually represents conditions that could be 5 or
10 degrees warmer than pre-industrial temperatures instead of just 1.5 or 2
degrees warmer,” said Dave Schimel, who supervises JPL’s Carbon Cycle and
Ecosystems group.
A half degree averaged out over the whole world can
mean much more of an increase in some locations and at certain times.
“There are places in the world where, for these important
breadbasket crops, they are already close to a thermal limit for that crop
species,” Schimel said. Adding to the burden, he said, “this analysis (the EGU
study) does not take into account the fact that pests and pathogens may spread
more rapidly at higher temperatures.”
And Schimel pointed out that heat can imperil agriculture
even when crops don’t die. “If you get really high temperatures or very dry
conditions during critical parts of the development of the crop, it produces
essentially no grain. For example, above certain temperature thresholds, corn
doesn't die but it doesn't grow seed. It doesn't grow a corncob. And other
crops are similar to that, where the development of the actual food part of the
crop is dramatically inhibited above critical temperatures.”
But what about that fertilization effect from carbon
dioxide? “It does help a bit, but it doesn't make the underlying problem go
away,” he said. “And by the way, if the plant was growing really fast when it
died, it still died.”
Can we avoid the extra half-percent temperature increase?
Schimel agrees that we should try hard to do so, but cautions that we don’t
know how to fine-tune global warming with that much precision. “If we aim for 2
degrees, we might hit 3 degrees,” he said. “If we aim for 1.5 degrees, we might
still hit 2 degrees.”
A multi-century commitment
Felix Landerer, who studies sea level and ice at JPL, said
timescale is critical to forecasting how high the ocean will rise.
“This paper looks at this century,” he said. “So the effects
appear to be fairly linear.” That is, a third more increase in temperature
produces about a third more increase in sea level.
“But,” he said, “I would frame the discussion in the context
that in recent studies—in particular of ocean-ice interactions—there is growing
concern that the ice sheets are very sensitive to the surrounding ocean
warming.” These studies show that giant glaciers in Greenland and Antarctica
melt not only from the top down, but also from the bottom up as relatively warm
ocean water makes its way to their undersides.
These studies show that giant glaciers in Greenland
and Antarctica melt not only from the top down, but also from the bottom up as
relatively warm ocean water makes its way to their undersides.
“At two degrees (of temperature increase),” he said, “you might
have crossed a threshold for significantly more sea-level rise than indicated
here.” In other words, even if we are able to limit the rise in global air
temperature to 2 degrees Celsius by the end of the century and stop the
increase at that point, the ocean holds so much heat that it can continue
melting ice sheets and thus raising sea level far beyond that point in time.
“The air temperatures level off, you (hypothetically)
stabilize them, but you have committed to sea-level rise over multiple centuries,”
Landerer said. “So it's good to stay away from two degrees. That's an
experiment you don't want to run. Because that experiment would potentially
wipe Florida off the map.”
Generations down the road
The EGU study found that the difference between 1.5 and 2
degrees Celsius of warming “is likely to be decisive for the future of tropical
coral reefs.” JPL’s Michelle Gierach was not surprised.
“Reef-building corals are extremely vulnerable to warming,”
she said. “Prolonged warming harms warm-water corals not only through bleaching
(a phenomenon in which corals under stress, such as from water that is too
warm, expel the algae they need to survive), but also through making them more
susceptible to disease.”
Gierach attended the international conference that produced
the Paris Agreement and she was happy to see the ocean and climate getting
their due attention. But she acknowledges the difficulty in turning that
attention into action over a long period of time.
“We want to see instant results. That's not something
that's going to happen with climate change. You need to just keep pursuing it
and know that generations down the road will reap the benefits.”
- Michelle Gierach, JPL climate scientist TOMADO D E ENVIO
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