Some headlines proclaimed recently that a particular day in July was the warmest in more than a 100,000 years. It is not scientifically possible to make such a claim. Here is why.
Temperature estimates from before thermometers were invented are derived from “palaeo proxies”. These are biological and chemical signatures of the temperature somewhere having been warmer or colder than a specific baseline temperature. Such a baseline is typically from the modern times, when thermometer records have existed. These measures are called “proxies” because they do not directly measure temperatures. Instead, they are simply the responses of physical, biological, and chemical processes to temperatures at that time having been warmer or colder than the baseline value.
Another thing we need to make claims about temperatures of a time in the past are some isotopes that undergo a steady rate of radioactive decay. Knowing this rate, and the expected quantity of the isotope X years ago, scientists can estimate how long it took to diminish to its present quantity. Based on the length of time one needs to go back to, the isotopes could be of carbon or lead, based on their half-lives (5,000 to more than 10 million years).
Longer and shorter timescales
A major assumption required to make the “paleo proxy” technique workable is that the processes that produced the proxies have operated similarly back then as they do today. More specifically, and crucially, for a period of hundreds of thousands of years, proxies – which are typically buried in the ocean and lake sediments – can only record temperature anomalies, i.e. deviations from the baseline, on time scales of centuries, if not thousands of years.
They are mixed by the ocean water above and the microbes within, smoothing out the information they contain over such long timescales. From this object, it is almost impossible to estimate even decadal or annual changes in long-term temperature, forget daily temperature.
Scientists derive estimates of temperature anomalies over shorter time scales from tree rings, corals, and the shells of marine and terrestrial organisms. But even here, the best of the “palaeo proxies” only provide weekly or seasonal timescale temperature anomaly estimates.
Similarly, in the spatial sense, all temperature proxies are only local or regional estimates of historical temperature anomalies. Reliable local temperature anomalies also come with fairly significant uncertainties – even for the Holocene epoch, the period in which we evolved as modern humans, that began in around 9700 BC. Global estimates, which are based on averaging all local proxies, have even higher uncertainties.
So there is simply no “palaeo proxy” that can give daily timescale temperatures.
The Holocene epoch
The most relevant bit of knowledge experts might wish to piece together today from historical temperature-related anomalies is whether any warming during the Holocene epoch can tell us something about the response of modern humans to climate change. There is some evidence as to the causes of demise of various civilisations in this epoch – and a climate-related event was not always the sole or even the proximal cause.
At the same time, modern humans’ (bipedal) ancestors also survived larger climatic changes over the evolutionary timescales of hundreds of thousands of years. The earth’s climate has witnessed glacials, or ice ages, and deglacials for at least a million years. The Holocene itself has been a deglacial period, with a relatively small volume of glaciers compared to a proper ice age.
The palaeoclimate serves as a rear-view mirror for the evolution of future climate, but only over longer timescales. Remembering that climate is what we expect and weather is what we get, a particular day of a particular year need not be the same at all to the same day from another year, simply as a result of small differences in temperatures, winds, humidity, rain, and so forth.
Global warming can produce record-breaking warm months and years – but we should be cautious about supposedly a record-breaking warm day, more so since even thermometer-based records are few and far between to make reliable claims of daily temperature records at the global scale.
Endangering climate action
Against this scientific background, what is the purpose of blaring headlines claiming that a particular day was the warmest in 100,000 years? It is scientifically impossible to estimate daily temperatures even for a particular day from last year – unless we have a thermometer measurement.
Are these headlines meant to scare people into changing their personal behaviour in order to reduce their individual contributions to climate change? Or are they expected to push governments to jump up and take action to mitigate climate change?
Perhaps they are all in the same vein as the widespread and persistent urge to report more and more alarmist claims without regard for their consequences. To wish to elicit collective and individual climate action while sacrificing scientific rigour and accuracy is a dangerous approach. It simply amounts to an ‘ the end justifies the means’ approach that is likely to lead to a loss of credibility for the climate community.
Modern societies have placed a considerable amount of trust in their scientists. Squandering this trust could render irreversible damage to the efforts that scientists and government officials have been making to improve global participation in climate negotiations, the willingness of governments to adhere to their climate commitments, and the grassroots initiatives that push governments and businesses into action, and to support communities dealing with the consequences of climate change.
Raghu Murtugudde is a visiting professor at IIT Bombay and an emeritus professor at the University of Maryland.