It has become a standard routine now to rank each month in terms of how warm it is compared to the same month in previous years in the instrumental record.
The most recent news to flash across all media was that March 2023 was the second warmest month on record. What does this actually mean in terms of impact on the planet, on the local weather, and on the human psyche? Do such headlines help or are they likely to render people numb to the idea of global warming by normalising the warming as a part of everyday life?
Many agencies across the world produce global climate anomaly reports regularly. The monthly report and the subsequent end-of-the-year annual summary by the U.S. National Oceanic and Atmospheric Administration serves as an excellent resource to contextualise the individual month’s ranking by temperature anomalies.
March 2023 was indeed the second warmest in the instrumental record. The warmest March occurred just a few years ago in 2016, when the biggest El Niño of the 21st century triggered a ‘mini’ global warming. But the January-to-March average temperature anomaly ranks 2023 as the fourth warmest such period on record. This raises obvious questions. Why was March 2023 the second warmest and not the warmest?
As seen in the figure below, each year’s March can be warmer or cooler than the March of the year before. Natural climate variability, including events like El Niño, can temporarily spike temperatures.
The old adage (often mistakenly attributed to Mark Twain) says that climate is what we expect and weather is what we get. In India, we expect March to be the beginning of the scorching summer season. But a particular year’s March may be cooler due to some other climate factors, such as a La Niña, and especially when averaged over a region as large as India or even an Indian state.
A year is an ‘El Niño year’ if warmer water spreads in a band from west to east over the equatorial Pacific Ocean. In a ‘La Niña year’, cooler water spreads east to west in the same region. Both phenomena have distinct and significant effects on the global climate.
(Global mean temperatures themselves represent the increasing amount of additional energy we are trapping in the earth system and preventing its escape to space by, among other things, increasing the atmospheric concentration of heat-trapping greenhouse gases.)
The distribution of temperature deviations for March 2023 from the baseline long-term average March temperature is visible in the global map of temperature anomalies (below). The monstrous warming to the west to north of India begins to tell the story of the weather anomalies that rendered a cooler March over Mumbai, excess pre-monsoon rains over the northwest, and scorching heatwaves in Kerala and Odisha.
The Arabian Sea has also warmed more than expected this March. We must watch carefully if this continues: it can favour a stronger monsoon but may also enhance cyclogenesis (i.e. birth of cyclonic circulation) over the Arabian Sea.
The global distribution of temperature anomalies is due to land-ocean-atmosphere processes that dynamically determine the weather and climate. Global warming does not mean each month or each year will be warmer than the previous month or the previous year.
Instead, a better place to begin would be by averaging the weather over a decade. Decade-to-decade warming clearly shows that humans are now ensuring each decade is warmer than the one before.
As with the temperature, precipitation anomalies for March 2023 show the impact of a warm March over Eurasia in the form of below-normal precipitation. We know that reduced snowfall over the Eurasian landmass has historically tended to favour a stronger monsoon. As it happens, 2023 is expected to be an El Niño year, and El Niños tend to produce weaker monsoons. So this summer’s El Niño effect could be blunted by the lower snow cover over Eurasia.
In sum, climate scientists need to provide the proper context when they compare and rank individual months against each other. This will help the people at large better understand global warming as well as its cascading effects on the weather that they experience every day.
All global warming is local; nobody lives in the global mean temperature. And the better people understand the impact of global warming in their backyard, the likelier they can be engaged in climate action.
Raghu Murtugudde is a visiting professor at IIT Bombay and an emeritus professor at the University of Maryland.