Global warming’s patterns are more important than its levels | Explained Premium

Did we really cross the 1.5 degrees C threshold in 2023? A new study adds fuel to the controversy. Raghu Murtugudde also explains that the patterns of global warming are more important than the level of warming at particular locations.

Many warming records were broken in 2023 along with climate disasters such as wildfires, cyclones, droughts, and floods. In this time, the focus of the public narrative – often with the participation of scientists – has often been on whether we crossed the ‘magical’ warming threshold of 1.5 degrees C. The best estimates, derived from data recorded by instruments, say the planet is just under this threshold.

But did we really cross the 1.5 degreesC threshold in 2023?

Before we discuss the answer, let’s remember that 1.5 degrees C is not a scientific threshold. It became enshrined in the Paris Agreement after intense negotiations by member-countries of the U.N. Framework Convention on Climate Change (UNFCCC). But it’s not a round number by accident: it comes from a figure – 2 degrees C – European politicians found easier to aim at in the 1990s.

Now a new study, published on February 5 in Nature, has added fuel to the fire of the threshold-crossing controversy. Based on estimates of warming from palaeo-thermometry, scientists from Australia and the U.S. have said that the earth’s surface has already warmed by more than 1.5 degrees C on average over pre-industrial levels. A major caveat of the study is that the scientists have collected warming data from only one location and have extrapolated it to be indicative of the global mean temperature trend.

This said, these so-called ‘palaeo proxies’ constitute an amazing technique that uses chemical evidence stored in various organic matter, such as corals, stalactites, and stalagmites, to approximate the temperature at some point in the past. But just as insightful as this chemical evidence can be, we should remember that it is still only indirect evidence of temperature changes with respect to a baseline temperature. The evidence can’t measure the actual overall temperatures.

Since palaeo proxies don’t directly measure the temperature, we call them proxies of past temperature deviations (the ‘palaeo’ denotes the past).

Researchers carefully calibrate the various chemical compounds assimilated by some species into their biogenic materials – such as calcium carbonate or chalk – in modernity to establish the relationships between those chemicals and the prevailing local temperature.