Northeastern researcher explains why climate models and observational data disagree
The findings are reported in the journal Nature Communications.
Scientific models have predicted that climate change will drive oceans in the Northern Hemisphere to warm faster than oceans in the Southern Hemisphere.
However, observational data over the last 70 years show the opposite — that Southern Hemisphere oceans are warming faster. New research from Northeastern University explains why.
“The climate models are too sensitive to greenhouse gases,” said Chengfei He, assistant professor of marine and environmental sciences at Northeastern.
As a result, He said, a crucial feedback loop involving wind speed, ocean water temperature and evaporation is overemphasized, having implications on tropical weather.
The findings were published in February in the journal Nature Communications, an open-access journal that publishes research in multiple areas of science.
According to the National Oceanic and Atmospheric Administration, climate models are based on well-documented physical processes such as how sunlight creates heat or how clouds form from moisture and simulate the transfer of heat, water and other factors through ocean currents and other processes through the climate system using complex mathematical equations.
These models can be checked by comparing their predictions with data that scientists collect to show what’s actually happening in the environment.
He explained that most models used to predict the effects of climate change are calculated using the average water temperature at the surface of the world’s oceans, a measure known as global mean surface temperature.
Those models have consistently agreed with scientists monitoring actual ocean temperature to show that mean surface temperature, as predicted, has increased by 1.5 degrees Celsius since the turn of the 20th century, He said.
But the global average of ocean temperature is not the right measure when looking at ocean warming in each hemisphere, He explained, because it cancels out the difference between the two sources of data.
For instance, if the Northern Hemisphere’s oceans average surface temperature is 10 degrees Celsius and the Southern Hemisphere’s oceans average 12 degrees Celsius, a global average of 11 degrees Celsius doesn’t reflect that there is a 2 degree difference in the temperatures in each hemisphere.
Instead, He said, you should look at what is called the interhemispheric temperature contrast, defined as the mean sea surface temperature difference between the Northern and Southern hemispheres.
Using the temperature contrast shows that conventional climate predictions for warming seas are wrong and confirm what is actually happening – the Southern Hemisphere’s oceans are warming faster than those in the Northern Hemisphere.
So what caused this discrepancy?
He cited a “very complicated” positive feedback loop that relies on trade winds, which are winds in the tropics that move predominantly from the east and curve towards the equator. Because of temperature differences between warming oceans and the air above the oceans these winds become stronger, which then means more water vapor rising from the ocean to the atmosphere, he explained. This in turn cools the ocean’s surface, which then intensifies the surface wind speed and increases evaporation even more, keeping the loop going.
This is where He’s research diverges from conventional models.
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Because the Northern Hemisphere has more land than the Southern Hemisphere, and land warms more than water, conventional models predict that greenhouse gases will warm the Northern Hemisphere more than the Southern Hemisphere, He explained. That warmth, the models predict, will weaken the trade winds in the Northern Hemisphere, prompting less evaporation from its seas and warming sea surface temperatures.
With weakened trade winds predicted in the Northern Hemisphere, the winds in the Southern Hemisphere are predicted to correspondingly strengthen, resulting in comparable cooler ocean temperatures on the southern half of the globe.
But He’s research finds that the greenhouse gases aren’t strengthening the feedback loop as much as the models predicted.
As a result, the wet weather and hurricanes forecast to result from the tropical rain belt — the area along the equator where the trade winds and the moisture it carries meet — will actually land to the south of where the climate models predict, He says, which is the opposite of what climate models predict. Areas now on the northern edge of the tropical rain belt, north of the equator, will receive less rain and be more at risk from drought and wildfires in the future; areas now just beyond the southern edge of the belt will become wetter.
“The tropical rain belt, hurricanes, drought, wildfires – they’re all related,” He said.
He hopes that future climate models, including ones that are scheduled to be released in the coming year, will incorporate his findings.
“We have to fix that to better predict tropical climate,” He said.
NOAA did not respond to a request for comment by press time.
