Thank a conifer for helping cut back on CO₂/Craig Medred photo

Those wet, windy and warm winters of El Niño that have too regularly transformed the dark months around the rim of the Gulf of Alaska into a drearier version of Seattle have a bright side, according to an international team of scientists studying the planet’s respiration.

They help suck carbon dioxide (CO₂) out of a CO₂-overloaded atmosphere.

“… Increased water availability and favorable temperature conditions (warmer spring and cooler summer) caused enhanced carbon uptake over North America near and during El Niño,” a team led by Lei Hu from the University of Colorado-Boulder reported in Climatology last week. 

The increase isn’t enough to balance out the human-produced CO₂ on the rise since the beginning of the Industrial Revolution, but every little bit helps.

In the process of photosynethsis, plants absorb CO₂ and release oxygen.
The leftover carbon is stored in tree trunks, stems and leaves.

Though the warm-water phase of the El Niño–Southern Oscillation (ENSO) flowing east across the tropics of the Pacific Ocean to the North America West Coast has long been known to influence global weather and climate, Hu wrote in the study, “the impact of ENSO on net terrestrial carbon fluxes in extratropical regions was thought to be small.”

Hu and almost two dozen colleagues from across the U.S. and in Canada and the Netherlands contend they have now found significant evidence to the contrary.

Upon examination of seasonal CO₂ samples from the U.S. and Canada, the group reported discovering “larger anomalies being derived for three biomes: forest field, crops, and coniferous forests.”

From the coastal rain forests of the Panhandle to the endless black spruce forests of the Interior, still-wild Alaska is a vast storehouse of those coniferous trees.

The study suggests El Nino influences on CO₂ uptake in such areas can increase carbon uptake enough to offset the influences of forest fires, which free carbon and release CO₂ into the atmosphere, and even some human activities.

A big CO₂ suck

“The natural variability of North American net ecosystem exchange resulting from climate variability largely associated with ENSO is much larger than the interannual variability of fossil fuel or fire emissions,” the study says. “It is also substantial compared to the magnitude of natural or anthropogenic (human) fluxes of greenhouse gases over North America:

“It is equivalent to roughly 80 percent of North American annual net ecosystem exchange, one-third of annual total North American fossil fuel CO₂ emissions, and twice as large as the total U.S. anthropogenic non-CO₂ greenhouse gas emissions (CH₄, N₂O, and fluorinated gases). This implies that shifts in climate (i.e., ENSO phases) have the potential to largely alter the overall carbon fluxes from North America.”

The authors said the reasons why plants suck more CO₂ out of the atmosphere in El Nino years vary regionally.

“To understand why North American net ecosystem exchange exhibits such a consistent response to ENSO, we first investigated how climate variables drive the interannual variability of North American net carbon uptake,” they wrote. “Current understanding of these relationships is so poor that projections of future carbon sequestration rates disagree in both sign (plus or minus) and magnitude. In particular, the relative roles of air temperature and water availability in controlling net ecosystem exchange have been intensely debated.”

Overall, the driving influence appears to be precipitation or lack thereof. But in some areas, indications are temperature is the controlling factor.

“…Warm conditions in spring enhance North American carbon uptake,” the study said; “whereas in summer (June to August), the opposite correlation exists, implying that increased summertime temperatures reduce terrestrial carbon uptake.”

Spring uptake would be keeping in line with a lengthening Alaska growing season that has benefitted from milder winters and earlier breakup in general for the last 50 years, but especially so during El Nino years.

The study also noted one little El Nino correlation of possible interest to Alaskans.

“In summer, it was cooler during El Niño than non–El Niño (neutral and La Niña) periods,” the study said. “Warmer than usual spring was observed during El Niño, especially over the boreal region (which covers Alaska)….

“The response of North American terrestrial ecosystems to ENSO derived (influence) is opposite to the ENSO–carbon flux relationship observed over tropical terrestrial ecosystems. In the tropics, the climatic impact of El Niño tends to be reduced precipitation and increased air temperatures (drought), whereas the imprint of El Niño over North America appears to be wetter with increased temperatures in winter and spring and decreased temperatures in summer.”

Whether that’s a good thing or a bad thing for Alaskans might depend on where in the state they live. Seventy degree days are considered something of a treat in coastal Alaska whereas days with temperatures of 80 degrees or above sometimes come too often for those in the state’s Interior.

In the record warm year of 2013, Fairbanks endured more than 30 days with temperatures at or above 80 degrees. Summer that year was warmer in Fairbanks than in northern Minnesota.

Don’t expect that to happen again this year if the correlations between El Nino’s and cooler summers continue. At the moment, the National Weather Service is predicting an existing El Nino will continue through the summer and into the fall.

The agency’s Climate Prediction Center puts the chance of a summer-long El Nino event at 70 percent fading to 55 to 60 percent in the fall.