People in the twenty-first century encounter no shortage of global warming news. For many of us, seeing the words "temperature tipping point" may rouse concern or alarm. For researchers who study Earth's climate, two factors help them know whether we're reaching a tipping point. One involves measuring carbon dioxide levels due to human activity, and the second monitors how plants absorb carbon dioxide, thereby mitigating its effects.

Life on Earth's landmasses currently mitigates about 30% of carbon emissions by human activity.1 Life in the oceans mitigates another 15%.2 Without this alleviation, more greenhouse gases would be pumped into the atmosphere and Earth's surface environment would be much warmer than it is.

As scientists seek to find a balance, they ask three questions: (1) Can Earth's terrestrial life mitigate more carbon emissions by human activity? (2) Is terrestrial life already doing the best it can possibly do to mitigate carbon emission by human activity? and (3) Is Earth's terrestrial life doomed to mitigate less and less carbon emission by human activity? A team of six ecologists and cyber system analyzers led by Katharyn Duffy sought answers and published their initial research findings in a recent issue of Science Advances.3

Biogenic Greenhouse Gas Mitigation
By far, the most significant greenhouse gas subject to mitigation by Earth's life is carbon dioxide. (Methane, the second most significant greenhouse gas regulated in part by life, is minor by comparison.)

This mitigation is determined by this formula: amount of carbon dioxide removed from Earth's atmosphere by photosynthesis minus the amount of carbon dioxide added to Earth's atmosphere by respiration. It's a key balance between photosynthesis and respiration. Both vegetation and animals respire carbon dioxide. Only cyanobacteria and vegetation photosynthesize. Currently, the net amount of carbon removed from Earth's atmosphere by terrestrial life is 2.6 petagrams (2.6 trillion kilograms) per year.4

Known major factors that affect biogenic greenhouse gas mitigation include surface temperature, carbon dioxide percentage in the atmosphere, droughts, and plant mortality. Increasing global mean (sum of all the values divided by the number of values) surface temperatures cause respiration to rise but the rate at which it rises slows down with increasing surface temperatures.5 For atmospheric carbon dioxide levels between 150 and 650 parts per million, rising atmospheric carbon dioxide levels enhance photosynthesis rates. For atmospheric carbon dioxide levels above 400 parts per million, rising atmospheric carbon dioxide levels hinder respiration but not in any major way until atmospheric carbon dioxide levels rise above 900 parts per million.6

Biogenic Greenhouse Gas Mitigation Measurements
Exactly how much increasing surface temperatures affect rates of respiration and photosynthesis has not been well understood. Scientists know that the terrestrial biosphere, as opposed to the oceanic biosphere, has been responsible for most of the changes observed so far. For this reason, Duffy's team focused on making accurate measurements of correlations between global mean temperature rise and terrestrial respiration and photosynthesis rates.

The team used data from the world's largest continuous carbon monitoring network, known as FLUXNET,7 to determine the temperature dependence of global rates of respiration and photosynthesis. They did a correlation analysis across 1,500 site years of the network of daily data from all major biomes (ecological communities) and plant functional types.

Their analysis revealed that photosynthesis rates peak at 18°C (64°F) and 28°C (82°F) for C3 and C4 plant systems, respectively. C3 plants comprise about 95% of Earth's plant biomass, including virtually all trees and food crops such as wheat, rice, barley, and soybeans. Only 8,100 plant species use C4 photosynthesis, but these species include sugar cane and corn.

Duffy and her colleagues noted that the global mean temperature of the warmest three-month period during the past decade has already exceeded 18°C. However, currently less than 10% of Earth's biosphere experiences mean temperatures greater than 18°C that result in the degradation of carbon uptake from the atmosphere. This limited degradation occurs because exposures in these regions to mean temperatures above 18°C typically are limited to two months.

The bad news from the analysis by Duffy's team is that a modest increase in the global mean temperature will cause a degradation of carbon uptake for biomes that cycle 40–70% of all terrestrial carbon. The rainforests of the Amazon and Southeast Asia and the taiga (subarctic) forests of Canada and Russia will be among the first biomes to exceed a temperature mean of 18°C for five months or more.

The research team ends their paper with a warning. Any further increase in the global mean temperature will cause a slow rise in terrestrial respiration rates and a sharp decline in rates of photosynthesis. The graphs they present in their paper imply that any increase in photosynthesis resulting from a higher carbon dioxide level in the atmosphere is likely to be more than offset by a decrease in photosynthesis as a consequence of the higher global mean temperature generated by the increase in atmospheric carbon dioxide.

Recommended Responses
How do we responsibly respond to such research findings? "Temperature tipping point" is in the title of the paper. For more than a decade the Paris Agreement and the Intergovernmental Panel on Climate Change (IPCC) have declared that a further global mean temperature increase of 1.5–2.0°C will put humanity past the tipping point where there will be no opportunity to prevent or recover from catastrophic economic, ecological, and health consequences. I present data and explanations in my book Weathering Climate Change8 that affirm the conclusions of the Paris Agreement and the IPCC.

Duffy's team's paper establishes that we are at the temperature tipping point now. While the kinds of economic, ecological, and health catastrophes I describe in Weathering Climate Change are not imminent, conditions will inevitably worsen with any measurable increase in the global mean temperature.

My concern, though, is that such news will generate alarm and panic and tempt political leaders to take strident action without full consideration of probable unintended consequences. In my view, we don't need to pass and enforce Draconian laws.

By providing strong economic incentives to stabilize the global mean temperature, people will perhaps take action quickly enough to prevent the tipping point revealed by Duffy's team and avert catastrophe. There are many solutions to global warming that can be implemented in the short term that will enhance the global economy, the world's ecosystems, and the health, wealth, and well-being of people everywhere.9

Endnotes

1. Trevor F. Keenan et al., "Recent Pause in the Growth Rate of Atmospheric CO2 Due to Enhanced Terrestrial Carbon Uptake," Nature Communications 7 (November 8, 2016): id. 13428, doi:10.1038/ncomms13428.
2. Keenan et al., "Recent Pause in the Growth Rate."
3. Katharyn A. Duffy et al., "How Close Are We to the Temperature Tipping Point of the Terrestrial Biosphere?" Science Advances 7, no. 3 (January 13, 2021): id. eaay1052, doi:10.1126/sciadv.aay1052.
4. Keenan et al., "Recent Pause in the Growth Rate."
5. Keenan et al.
6. Astrid C. Wittmann and Hans-O. Pörtner, "Sensitivities of Extant Animal Taxa to Ocean Acidification," Nature Climate Change 3 (August 2013): 995–1001, doi:10.1038/nclimate1982; Hugh Ross, "Complex Life's Narrow Requirements for Atmospheric Gases," Today's New Reason to Believe (July 1, 2019).
7. Gilberto Pastorello et al., "The FLUXNET2015 Dataset and the ONEFlux Processing Pipeline for Eddy Covariance Data," Scientific Data 7, no. 225 (July 9, 2020), doi:10.1038/s41597-020-0534-3.
8. Hugh Ross, Weathering Climate Change (Covina, CA: RTB Press, 2020), 23–46, 187–98.
9. Ross, Weathering Climate Change, 199–218.