Editor’s note: The following was written by Rebecca Lindsey and Tom Di Liberto with the National Oceanic and Atmospheric Administration.
We’ve been talking about La Niña — the cool phase of the El Niño-Southern Oscillation climate pattern — for going on three years now. People have started to ask us whether all these La Niñas could offset global warming.
The short answer is no. La Niña is no match for global warming.
The simplest evidence is that global average temperature during recent La Niña years is warmer than for El Niño years in earlier decades. In fact, the La Niña year of 2020 tied 2016 — a year that started with a major El Niño — as the all-time-record-warm global-surface temperature.
But if global warming continues year after year regardless of La Niña, then how does the surface temperature manage to cool at all during La Niña? Where does that excess heat go?
We posed these questions to Michael McPhaden, an oceanographer and expert on El Niño at the NOAA Pacific Marine Environmental Laboratory, Richard Allan, a climate scientist at the University of Reading, and Shang-Ping Xie, a climate dynamicist at the University of California-Scripps Institution of Oceanography.
QUESTION: When we look back over the historical temperature record, we see that the warmest years in any decade are usually El Niño years and the coldest years are usually La Niña years. Why is that?
MCPHADEN: During El Niño, unusually warm sea-surface temperatures in the central and eastern tropical Pacific Ocean lead to increased evaporation and cooling of the ocean. At the same time, the increased cloudiness blocks more sunlight from entering the ocean. When water vapor condenses and forms clouds, heat is released into the atmosphere.
So during El Niño there is less heating of the ocean and more heating of the atmosphere than normal.
During La Niña, the opposite happens. With colder La Niña sea-surface temperatures, there is less evaporative cooling of the ocean, less convective cloudiness blocking the sun from heating the ocean and less convective heating of the atmosphere.
So the atmosphere warms more during El Niño and less during La Niña, and that affects global average temperature. … It may not sound like much but it involves huge transfers of heat between the ocean and atmosphere that affect the entire globe.
XIE: Outside the tropics, the atmospheric circulation change during La Niña causes warming over much of the North Pacific with cooling over Alaska and western Canada. But the surface-temperature effect is small when averaged around the globe outside of the tropics. Thus the La Niña contribution to global-average surface temperature is mostly from the tropics.
QUESTION: Satellite and ocean observations show an increase in energy building every year as greenhouse gas concentrations continue to increase, even through periods when the rate of surface-temperature increase slows. That’s largely because the deeper layers of the ocean are storing much of the excess heat. What do we know about where and how the ocean stores this heat?
MCPHADEN: On the seasonal and year-to-year time scales that characterize ENSO variations, it’s the upper few-hundred meters of the ocean that matter most for the storage, transport and exchange of heat with the atmosphere. The tropical Pacific is the main center of action on those time scales, though there is a partial compensation of tropical heat gain and loss from opposite tendencies at higher latitudes.
For example, during El Niño the tropical Pacific experiences a large heat loss to the atmosphere, but some of that heat is reabsorbed into the ocean at higher latitudes because of El Niño teleconnections.
On longer decadal time scales, like that of the hiatus in global surface warming during the first decade of the 21st century, the tropical Pacific experienced a prolonged period of unusual heat gain because of the La Niña-like conditions that prevailed at this time. Much of that heat gain was transported to the Indian Ocean through the Indonesia Seas — the porous boundary that separates the Pacific from the Indian Oceans.
Outside the tropics, sinking of water masses in the high latitudes of the North Atlantic can transport heat to abyssal depths on decadal and longer time scales. The Southern Ocean is also a key region for the uptake of anomalous heat associated with climate change.
Where the ocean exchanges heat with the atmosphere, and how the ocean stores and transports that heat, is really central to our understanding of how the climate system works across all time scales. To riff on the expression “follow the money,” if you want to understand how the climate system works, you need to “follow the heat” in the ocean.
QUESTION: Richard wrote in 2014 about changes in ocean circulation across the Pacific that were likely responsible for increased deep-ocean heat uptake and slower surface warming during the first decade or so of the 20th century. Since then we’ve had another shorter period since 2016 where global surface temperatures have not increased noticeably. Are the same processes at work?
ALLEN: Warming has continued apace since 2014. The recent 2011-20 decade was about 1.1 degrees C warmer than the 1850-1900 early-industrial period based on a recent United Nations-Intergovernmental Panel on Climate Change report.
But the warming doesn’t progress smoothly — it comes in fits and starts. For example, rapid warming since about 2011 peaked with the 2015-16 El Niño event. That level of warmth has been maintained since rather than continuing to rise, with further ups and downs balancing out. Since 2014 energy has continued to accumulate and heat the oceans.
Surface temperature is closely related to heating of the uppermost layer of ocean, so how much of the planetary heating goes into this upper layer or layers beneath affects the surface warming rate.
In a simple sense, more overturning circulation in the Pacific — what goes down must come up — can draw down warm water in one region; that’s replaced with cooler sub-surface water in another region. More heat can be used in warming that newly emerged cool water.
The upshot is that accumulating energy is effectively moved downward into the ocean and not concentrated in the upper layer, temporarily suppressing the rate of surface warming.
What has been learned since 2014 is that global warming is progressing with an unexpected pattern — more warming in the west Pacific and less in the east Pacific than simulated on average by global climate models. Whether that’s a temporary effect or a longer-term discrepancy remains to be seen, but ultimately the long-term warming due to greenhouse gas increases will dominate.
MCPHADEN: Climate-change skeptics latched onto the hiatus in global surface warming in the first decade of this century to argue that the science was flawed and that we didn’t need to be concerned after all. We’ve learned a lot more since then about how natural variability, and in particular ENSO, affects the global energy balance and its signature in global mean surface temperatures. As a result, this most recent slowdown has stirred much less controversy, which is a real measure of progress in effectively communicating scientific advances.
XIE: Following the major El Niño of 2015-16, the global surface temperature increase has slowed. During the past summer, the anomalous cooling of the equatorial Pacific intensified and the current La Niña formally entered a third year, the first such long-haul event in the 21st century.
The ongoing La Niña may prevent global average temperature from breaking the record in 2023, but greenhouse-gas-
induced global warming grows steadily in magnitude. In fact it most likely helped 2020, a year of La Niña, to tie the all-time record temperature of 2016, a year following a major El Niño.
LINDSEY: Although the comparison is simplistic, we can also see that La Niña is no match for global warming simply by the relative sizes of their effects. Global warming due to human- produced greenhouse gases has increased Earth’s average surface temperature by about 2 degrees Fahrenheit during the past 140 years. Even a strong La Niña only cools global average surface temperature by a couple-tenths of a degree Fahrenheit.
