Modeling what would happen to the UK if the Gulf
Stream shuts down
A new study compares climate warming to climate
warming + a tipping point.
JOHN TIMMER
- 1/14/2020, 3:26 PM
While we
track climate change as a gradual rise in temperatures, most of its effects are
going to be anything but gradual: an increased risk of extreme temperatures and
storms, extended droughts, expanded fire seasons, and so on. There's also the
risk of pushing the climate past some tipping points, which can change the
state of entire areas of the globe. But it can be difficult to understand the
impact of tipping points, given that they're occurring against a backdrop of
all those other climate changes.
For
example, one of the major potential tipping points we're aware of is the
shutdown of the North Atlantic's current system, which brings warm water north,
moderating the climate of Europe. The loss of this warm water would obviously
result in a cool down in Northern Europe. But calculations indicate that the
shutdown isn't likely to take place until after the planet had warmed enough to
offset this cooling.
But
temperatures aren't the only thing affected by some of the tipping points we've
looked at. And a new study manages to separate out the effect of shutting down
the gulf stream from the general impact of a warming climate. And it finds
that, for the UK, changes in precipitation may have a larger impact than
changes in temperature.
Tipping the
AMOC
What's a
climate tipping point, and why do we think the Gulf Stream can be tipped off?
Tipping points exist where a series of changes driven by warming act to
reinforce each other, making it difficult to reverse them. We covered one back
in 2018, describing how a body of cold water in the Arctic's Barents Sea kept
warmer Atlantic Ocean water out of that basin. But, with the warming climate,
the cold water gradually vanished, allowing Atlantic water to invade the
Barents Sea. Since that water is also relatively warm, the change is going to
be exceedingly difficult to reverse.
As for the
Gulf Stream, it's part of a larger system called the Atlantic Meridional
Overturning Circulation, or AMOC. This system brings warm water from tropical
areas north on the surface. Eventually, in and near the Arctic, it cools and
sinks, traveling south along the ocean bottom.
All of this
depends on the waters near the Arctic cooling and sinking. But that's not
guaranteed in a warming world. Our warming climate will eventually get rid of
the sea ice, allowing the ocean waters to absorb more sunlight and exchange
heat with the atmosphere. It will also melt more ice, decreasing the saltiness
and making it less likely to sink. Combined, these factors can ultimately stop
the circulation from overturning, taking the Gulf Stream with it. And, once it
stops, it's not a simple thing to re-start, since warm water will effectively
pool up on the surface and stay there, keeping the sea ice from regrowing.
Currently,
climate models indicate that the Gulf Stream will weaken but won't shut down
entirely this century, if ever. But the research team behind the new paper
decided to model what might happen if we saw a shutdown in the middle of this
century, since it provided a relatively easy way of separating the shutdown's
effect from the impacts of a warming climate. To limit the scale of that task,
they focused on the climate of the UK and its impacts on agriculture.
Colder and
drier
The work
started climate model runs and sampled what happened in both 2020 (aka the
present) and 2080. Some runs were allowed to run on to 2080 with no changes,
other than a midrange level of ongoing greenhouse gas emissions. Others were given
what the authors term a "freshwater hosing" from 2030 to 2050, which
shut down the Atlantic circulation. Those were then allowed to continue on to
2080 with changes only in greenhouse gas levels. By comparing the two—warming,
and warming plus AMOC shutdown—they are able to separate out the impact of the
changes in ocean currents.
Under a
simple warming scenario, the UK is small enough that it warms pretty evenly. By
2080, this leaves it with increasing heat in the south, with a warmer but still
moderate Scotland. This leads to a somewhat reduced rainfall in the south,
partly offset by increased rain in the Scottish highlands. On average, the
model predicts a rise of 1.9°C, and a drop of 20mm in the average rainfall; the
authors call the latter change "modest."
Turning to
agriculture, the researchers find that the temperature change will make a
significant amount of additional land viable for agriculture. Many regions
won't need additional water for this to work, and in others, a small addition
of irrigation will be sufficient. At current prices, they find that the cost of
building and maintaining the irrigation infrastructure more than offsets the
profit from the additional productivity, assuming prices scale with inflation.
But, they
note, many analyses indicate that prices are likely to scale faster than
inflation, so irrigation may end up being a viable option. If so, irrigation
could shift the UK from a situation where 15 percent of its arable land is
rainfall-limited to one where the total arable land area rises from 32 to 42
percent.
Things are
quite a bit different if the AMOC shuts down. Rather than rising, temperatures
would actually drop by an average of 3.4°C. That drop would occur on a
gradient, with northern Scotland cooling the most and southern England seeing
the least impact and therefore seeing conditions similar to what it currently
experiences. But, more dramatically, rainfall during the growing season is
expected to drop by 123mm. That drop is enough to reduce the UK's percentage of
arable land from 32 percent to just seven percent. Obviously, this would cause
a big hit to the UK's agricultural productivity. Irrigation could again offset
this, but the scale of the changes needed would be far larger; the authors
estimate adding this irrigation at ten times the value of the crops that would
be produced. But they note that it's not clear if the UK would have enough
water to spare to fully reverse the loss of rain.
The
underlying scenario here—the complete shutdown of the AMOC and thereby the Gulf
Stream by midcentury—is likely to be science fiction. But the work indicates
that one of the ideas about what would happen isn't: Europe really would cool
down enough to more than offset the warming climate by the end of the century.
But, in terms of food production, this is almost an afterthought—the changes in
rainfall are far more significant. What's needed next is an analysis of what
would happen if, instead of a complete shutdown, the expected gradual reduction
took place.
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