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.

Nature Food, 2019. DOI: 10.1038/s43016-019-0011-3 (About DOIs).