EarthTalk…Questions and Answers About Our Environment

May 2021

Dear EarthTalk: How does climate modeling work? What is the state-of-the-art in the field these days, and what do these most recent models tell us about our future?        ~ J.M., Austin, TX

Climate models are 3D figures of the Earth’s surface that demonstrate the cycling of energy and materials through the atmosphere, oceans and land. They compile geography, physics, chemistry and biology to analyze historical data and predict future global conditions. Ultimately, these models allow researchers and the public to explore Earth’s systems, climate change and the impacts of human activity on the planet.

The paint-like swirls of this visualization from Los Alamos National Laboratory depict global water-surface temperatures, with the cooler temperatures designated by blues and warmer temperatures by reds, as delineated by recent climate modeling efforts. Credit: Los Alamos National Laboratory, FlickrCC.

The Coupled Model Intercomparison Projects (CMIP), conducted by the Intergovernmental Panel on Climate Change (IPCC), are some of the most detailed and expansive climate models to date. They show that the warming predicted from this century may be 0.4 degrees C greater than what was deduced from the CMIP5 in 2013. This may not sound significant, but it takes an immense amount of heat and, in this case, trapped greenhouse gases to warm the atmosphere, oceans and land that much. In the 1700s, it only took a 1-2 degree drop in global temperature to plunge the Earth into the Little Ice Age.

The CMIPs have also been proven to be very accurate. Fourteen of the 17 models done between 1970 and 2007 made similar projections of temperature change through the next decade as found by UC Berkeley researchers. These findings have bolstered the support for and confidence in the use of climate models.

Climate models may be portrayed as alarmist or extreme by some skeptics, but they actually tend to be more conservative with their predictions since a net positive feedback—that is, an overloading of the system with greenhouse gases—skews toward a stronger climate response. One example is the recorded sea level rise from satellite data collected from 1993 to 2008. CMIP3 models predicted just 1.9 millimeters of sea level rise while the data collected in the following years showed 3.4 millimeters of sea level rise. CMIP models also underestimated sea ice melt-out rates. Between 2007 and 2009, the amount of sea ice that melted was 40 percent greater than the average predicted by CMIP4 models.

One aspect of the new CMIP6 models that will work toward addressing underestimations are their higher climate sensitivity in comparison to past assessments. Climate sensitivity is the amount of long-term warming expected after a doubling of carbon emissions that greatly affects the model’s predictions. To eliminate more of the uncertainty from past models, scientists and mathematicians are constantly evolving climate models to improve their accuracy.

Though it is undeniable that models innately have uncertainties and the Earth encompasses a collection of complex systems, climate models have proven to be reliable predictors of climate change trends. Whether or not these most recent models will continue in that vein can only be determined over time, but climate action is not something that can wait any longer.

CONTACTS: “CMIP6: the next generation of climate models explained,”; “Newest climate models shouldn’t raise future warming projections,”; “Some of the latest climate models provide unrealistically high projections of future warming,”

Dear EarthTalk: Besides generating seasonal allergens, do any plants actually reduce air quality or cause air pollution?          ~ Mike T., San Juan, PR

While many of us thought Ronald Reagan sounded crazy back in 1981 when he told America that “trees cause more pollution than automobiles do,” the then-President may have been on to something.

Researchers from the University of California at Berkeley recently unveiled the results of a study in which they determined that certain trees and plants common in Southern California off-gas natural yet nevertheless harmful volatile organic compounds (VOCs)—to attract pollinators, protect against environmental stressors, and repel herbivores—especially during the hottest months of the year. This so-called particulate matter pollution is not only dangerous in and of itself to breathe in as the tiny molecules can get lodged in the lungs, but it also leads to the formation of ground-level ozone and smog.

While Reagan’s comment may have been an utter falsehood back then, who knew it would take the electrification of the transport sector to make it actually ring true today? The only way researchers could have any way of knowing that these plants are a significant contributor to air pollution would be by the elimination of the vast majority of fossil-fuel-derived “background noise” that we are no longer subject to as we breathe in the air around the streets of California—where the nation’s strictest automotive fuel efficiency standards have driven many to Teslas, Bolts, Volts, Leafs, Polestars, e-Trons, Priuses and other green rides—and elsewhere.

The researchers found that over the past two decades, concentrations of these VOCs fell by 50 percent between 1999 and 2012, and then to undetectable levels during the cooler months thereafter. But when the mercury rose, even without additional automotive emissions, so did concentrations of airborne VOCs. Four out of five excessive heat days (with air temps topping 100℉) led to unsafe VOC levels outside.

Palms are among several tree species that off-gas natural but potentially harmful air pollution when the weather heats up. Credit: Pixabay.

With transportation emissions off the hook as the culprit, researchers looked to the plant community for answers. One of the worst offenders is the iconic and ubiquitous fan palm, but sycamores, poplars, willows and many oaks and pines also off-gas their fair share of VOCs when the weather heats up — which will be happening more frequently as we warm the atmosphere with greenhouse gases. Backyard planners and landscape designers concerned about air quality and the environment might want to steer clear of these species when choosing plants.

“I am not suggesting that we get rid of plants, but I want people who are thinking about large-scale planting to pick the right trees,” says Ronald Cohen, the Berkeley atmospheric chemist who led the research effort. “They should pick low-emitting trees instead of high-emitting trees.” If you have the luxury of choosing which trees to plant and/or replace, and you care about your community’s air quality, some good choices include alders, magnolias, manzanitas, birches, hazelnuts, gingkos, apples and elms.

CONTACTS: “With drop in LA’s vehicular aerosol pollution, vegetation emerges as major source,”; “Urban trees and ozone formation,”

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