While we track climate change as a gradual increase in temperature, most of the effects will be anything but gradual: increased risk of extreme temperatures and storms, prolonged droughts, longer fire seasons, and so on. There is also the risk of pushing the climate past some tipping points, which could change the state of entire parts of the world. But it can be difficult to understand the impact of tipping points as they occur against the backdrop of all those other climate changes.
For example, one of the major potential tipping points that we are aware of is the closure of the current North Atlantic Ocean system, which brings warm water northwards and moderates Europe’s climate. The loss of this warm water would of course lead to cooling in Northern Europe. But calculations indicate that the shutdown is unlikely to happen until after the planet has warmed enough to compensate for 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 the effect of closing the Gulf Stream from the overall impact of a warming climate. And it finds that, for the UK, changes in precipitation could have a greater impact than changes in temperature.
Tipping the AMOC
What is a climate tipping point and why do we think the Gulf Stream can be tipped? There are tipping points where a series of changes driven by warming reinforce each other, making it difficult to reverse them. We covered one in 2018, describing how a body of cold water in the Barents Sea kept the warmer waters of the Atlantic Ocean out of that basin. But with the warming climate, the cold water gradually disappeared, allowing Atlantic water to enter the Barents Sea. Since that water is also relatively warm, it will be extremely difficult to undo the change.
As for the Gulf Stream, it is part of a larger system called the Atlantic Meridional Overturning Circulation, or AMOC. This system brings warm water from tropical regions to the north to the surface. Eventually, in and near the Arctic, it cools and sinks south along the ocean floor.
All this depends on the cooling and sinking of the waters near the Arctic. But that’s not guaranteed in a warming world. Our warming climate will eventually lose sea ice, allowing ocean waters to absorb more sunlight and exchange heat with the atmosphere. It will also melt more ice, decreasing its salinity and making it less likely to sink. Combined, these factors can ultimately keep the circulation from tipping over and taking the Gulf Stream with it. And once it stops, it’s not easy to start again, as warm water will effectively collect on the surface and stay there, preventing the sea ice from growing back.
Currently, climate models indicate that the Gulf Stream will weaken, but not completely stop 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 mid-century, because it was a relatively easy way to separate the effect of the shutdown from the effects of a warming climate. To narrow the scope of that task, they focused on the UK’s climate and its effects on agriculture.
Colder and drier
The work began by performing climate model runs, sampling what happened in both 2020 (aka the present) and 2080. Some runs were allowed to continue through 2080 with no changes except for an average level of sustained greenhouse gas emissions. Others received what the authors call a “freshwater hosing” from 2030 to 2050, which halted Atlantic circulation. Those were then allowed to continue until 2080 with only changes in greenhouse gas levels. By comparing the two — warming and warming plus AMOC shutdown — they can separate the impact of the changes in ocean currents.
In a simple warming scenario, the UK is small enough to warm quite evenly. In 2080 this will lead to increasing heat in the south, with a warmer but still temperate Scotland. This leads to somewhat reduced rainfall in the south, partly offset by increased rainfall in the Scottish highlands. On average, the model predicts an increase of 1.9°C and a decrease of 20 mm in average rainfall; the authors call the latest change “modest.”
As for agriculture, the researchers find that the temperature change will make a significant amount of additional land viable for agriculture. Many regions do not require additional water for this, and in others a small addition of irrigation is sufficient. At current prices, they find that the cost of building and maintaining the irrigation infrastructure more than offsets the gains from the extra productivity, assuming prices grow with inflation.
But, they note, many analyzes indicate that prices are likely to rise faster than inflation, so irrigation may eventually become a viable option. If so, irrigation could shift the UK from a situation where 15 per cent of arable land is limited by rain, to a situation where the total agricultural area increases from 32 to 42 per cent.
Things are a little different when the AMOC shuts down. Instead of rising, temperatures would actually fall by an average of 3.4°C. That decline would take place on a slope, with northern Scotland cooling the most and southern England having the least impact and therefore seeing conditions similar to what it is currently experiencing. More dramatically, though, rainfall is expected to decrease by 123mm during the growing season. That drop is enough to reduce the percentage of arable land in the UK from 32 percent to just seven percent. Obviously this would be a major blow to UK agricultural productivity. Irrigation could again compensate for this, but the scale of the changes needed would be much larger; the authors estimated adding this irrigation to ten times the value of the crops that would be produced. But they note it’s not clear whether the UK would have enough water left to completely reverse rain loss.
The underlying scenario here — the complete closure of the AMOC and thus the Gulf Stream by mid-century — is likely science fiction. But the work indicates that one of the ideas about what might happen isn’t: Europe would actually cool 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 much more significant. What is then needed is an analysis of what would happen if, instead of a complete shutdown, the expected gradual decline happened.
Nature Food2019. DOI: 10.1038/s43016-019-0011-3 (About DOIs).