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 degrees C 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.
Measuring the crossing
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.
When such a biogenic material from the past is found, scientists can piece together when the biogenic material was deposited (using the quantity of certain isotopes that decay at a steady rate over time). They then study the assimilated chemicals to deduce the temperature deviations during that time period. The results are very local temperature anomaly estimates from the past, so they can’t be the basis for any scientifically robust claims about tiny deviations of past temperatures from instrumental records.
Next, let’s examine the significance of crossing the 1.5 degrees C threshold. The expositions that made this claim – articles, headlines, etc. – do not really offer any clear explanation of why such exceptional warming occurred or how it can explain some location-specific disasters.
Even worse, no one has been able to fully explain the patterns of warming associated with this threshold exceedance or, in fact, the pattern of any level of global warming. This is important because warming patterns matter the most for our ability to manage the disasters associated with global warming.
Even the amount and the distribution of the 2023 monsoons have not been explained yet, and it is unclear how the combination of the El Niño, its unusual pattern, and global warming conspired together to produce the monsoons India experienced. Note also that the Indian subcontinent experienced significant cooling during 2023.
We will be ignoring the lessons we have learned from various El Niños and their impacts on global warming and on the monsoon if we continue to focus overmuch on the extent of global warming rather than its spatial variations and how they evolve.
El Niño as a warming paradigm
The tropical Pacific Ocean keeps soaking up heat during normal and La Niña years and belches it out in an El Niño year. This causes a mini global warming, with consequences at distant locations – the so-called teleconnections; the latter is what tells us that the pattern of warming really matters.
Depending on whether warming due to an El Niño is in the eastern Pacific Ocean or closer to the international dateline, the impacts on the monsoon and the other parts of the world can be very different.
An added difficulty is that the El Niño teleconnections themselves also modify the warming pattern. As a result, droughts can have a stronger feedback that affects temperature than floods in many instances. The massive deluges that California is experiencing this winter are driven by the El Niño but likely amplified by global warming. The amounts of rain and snow will subsequently feedback to local and global temperatures.
These same processes work in the global warming regime as well. As human-made greenhouse gases initiate the warming, the warming pattern is amplified in the Arctic and over the desert regions of the Middle East but damped over the eastern Pacific and the northern Atlantic oceans. These local warmings and coolings and their magnitudes determine the net effect of natural variability and global warming in a particular locality.
Need for accurate predictions
It is hardly possible to over-emphasise our need for accurate predictions of warming patterns to manage the climate change impacts that have already landed. We need such predictions not only on an event-to-event basis but also in order to adapt to the changing seasons and the unavoidable harm they are rendering to lives, livelihoods, and economies.
In sum, we will be better served by keeping the global warming patterns in mind when arbitrary thresholds lacking in any real scientific basis threaten to sweep us away.
- The best estimates, derived from data recorded by instruments, say the planet is just under this threshold.
- Now a new study, published on February 5 in Nature, has added fuel to the fire of the threshold-crossing controversy.
- The tropical Pacific Ocean keeps soaking up heat during normal and La Niña years and belches it out in an El Niño year.
Raghu Murtugudde is a visiting professor at IIT Bombay and an emeritus professor at the University of Maryland.
