The Intergovernmental Panel on Climate Change has repeatedly warned that allowing global temperatures to increase by 1.5C above pre-industrial levels would have a devastating and irrevocable impact on our climate. But what, theoretically, would happen to Earth if the planet were to cool by the same amount? Simon Salter, Flintshire
Send new questions to nq@theguardian.com.
Readers reply
An interesting question, Simon, and one for which we have recent (in geological terms) experience. The little ice age, which ran from roughly 1300 to 1850, saw global temperatures decline by 0.6C, but at its peak it was more than 1C cooler and more than 1.5C cooler than today. Although not enough to set off a full-blown ice age, the little ice age saw much harsher winters throughout North America and Europe, with the Thames freezing over regularly, alpine glaciers advancing into high-elevation villages in Switzerland and the failure of the Greenland Norse, as growing crops in sheltered fjords became impossible. Inuit following ice flows even showed up off the coast of Scotland on a few occasions.
In short, it probably wouldn’t be enough to kick off a new ice age, but would be enough to make winters in Britain much harsher (or fun, depending on how much you like ice hockey); great for ice skating in the Netherlands and skiing in the Alps, but not ideal for Greenlanders. Dr Stephen Hart, lecturer, faculty of natural resource management, Lakehead University, Canada
Well, not normally one to tip a big box of marbles on to the dance floor, but I have read a couple of things from more eminent skulls than my own concerning the stability of the Gulf Stream. Now, if that packs up … bricklayersoption
Should the Atlantic Meridional Overturning Circulation (AKA the Gulf Stream, although it isn’t the same thing exactly) collapse, we in the UK, as well as many other countries of northern Europe, would find out soon enough what a substantial reduction in our mean temperature feels like. That doesn’t mean that the rest of the world won’t be frying to a crisp. Oradour
Why do we think this is something that the general public can opine on? The fact that this has been posed to us is a fantastic example of the post-truth, post-science, everyone-is-qualified-to-talk-about-every-subject society in which we live. I don’t care what Joe Bloggs who “once read a book about science stuff” thinks might happen if the world were to cool down by 1.5C. I care what actual experts in the field think. Please could you interview one of them instead? deparfitt
The impact on human civilisation, whether 1.5C above or 1.5C below pre-industrial times, would be the same: a disaster. Our cities are designed based upon the relative stability of the Holocene epoch, when fresh water and arable land could be relied upon enough to settle down and plant roots – literally. Significant shifts, as we are seeing with global heating, play havoc with water supplies and the ability of the land to produce.
For the natural world, it is a bit harder to say. We know that warming threatens the very fundamentals of biodiversity; it could be the collapse of highly temperature-sensitive phytoplankton, the death of reef systems or changes in soil geochemistry that cause major disruptions in the food chain. The natural world evolved in the same narrow band of stability that we did. With warming or cooling, it may not be that resilient. DekeDeke
Earth is technically in an ice age right now – the Quaternary glaciation – and has been for 2.5m years. We don’t notice because, for the past 11,000 years, we have been in an interglacial period, where the ice sheets have retreated to the poles. If Earth became 1.5C colder on average, perhaps through massive vulcanism or nuclear war, it is likely that this would bring the interglacial period to an end. This is because there are a number of feedback mechanisms that could be triggered by a drop of 1.5C (just as there are for a rise of 1.5C).
For example, a colder planet would have more shiny, white areas that tend to reflect sunlight back into space, further reducing the temperature in those areas, encouraging more ice and snow to form. The end result is that much more of the planet gets covered in ice. In the previous glaciation, 3km-thick ice sheets extended as far south as Turin, making the UK, as well as much of northern Eurasia and North America, uninhabitable.
Of course, that is now unlikely to happen. Feedback mechanisms driven by our greenhouse gas emissions are pushing temperatures ever higher. If we don’t act, we could bring about the end of the Quaternary and the start of an ice-free greenhouse world, in which most of today’s cities would be 50 metres under water. SemiFunctional
Bernie Sanders would be a billionaire due to royalties from his mitten empire. JonLocke
