The headline at Science Daily is that wildfires and other burns lead to climate change. The paper itself asks: “As such, particle burn-off of clouds may be a major underrecognized source of global warming.” For me what matters are the deaths in the here and now:
“We calculate that 5 to 10 percent of worldwide air pollution mortalities are due to biomass burning,” Jacobson said. “That means that it causes the premature deaths of about 250,000 people each year.”
This is similar to Indur Goklany’s conclusion in 2011:
In a study published in Journal of American Physicians and Surgeons, Indur Goklany calculated the additional mortality burden of biofuels policies and found that nearly 200,000 people died in 2010 alone, because of efforts to use biofuels to reduce CO2 emissions.
Goklany (2011) estimated that the increase in the poverty headcount due to higher biofuel production between 2010 and 2004 implies 192,000 additional deaths and 6.7 million additional lost DALYs in 2010 alone.
He compared this death tally to the WHO figures for deaths attributed to global warming and finds that the biofuels policies are more deadly. (And he is not including any increase in poverty due to other anti-global warming practices).
What rather matters is how much of this is “wild” fire and how much is agricultural fire. Indur Goklany’s work looked specifically at biofuels, so related to man-made air pollution. From the actual Jacobsen paper it’s clear that this is mostly thought to be man-made fires:
Seiler and Crutzen  further estimated that wildfires in temporal plus boreal forests comprised ~35.7% of all dry matter burned but pointed out that 70–90% of such wildfires were due to human activity (e.g., campfires, debris burning, cigarettes, etc.). Thus, of total world fire emissions today, ~7.1% (3.6%–10.7%) may be natural and the rest, anthropogenic. Houghton  indicates that, in 1850, CO2 emissions from land use change may have been ~34% those in 2005. Thus, in 1850, BB [Burning Biomass] emissions were lower than those today. Such emissions may have been mostly anthropogenic [e.g., Marlon et al., 2008] although partly natural as well. Today, BB emissions are much higher with only a small percent natural.
Does black carbon change clouds?
The study goes on to say that black carbon effectively causes global warming too because it heats water droplets, melts clouds and ice, and reduces the thickness of cloud cover.
So any minute now I expect environmentalists around the world will start a “Boycott Biomass” campaign. Clearly any truly compassionate green could do nothing less since biomass is a net killer, and warms the planet too.
[Science Daily] “But Jacobson’s research also demonstrates that it isn’t just the CO2 from biomass burning that’s the problem. Black carbon and brown carbon maximize the thermal impacts of such fires. They essentially allow biomass burning to cause much more global warming per unit weight than other human-associated carbon sources.
Black and brown carbon particles increase atmospheric warming in three ways. First, they enter the minuscule water droplets that form clouds. At night, that’s not an issue. But during the day, sunlight scatters around within clouds, bathing them in luminescence.
When sunlight penetrates a water droplet containing black or brown carbon particles, Jacobson said, the carbon absorbs the light energy, creating heat and accelerating evaporation of the droplet. Carbon particles floating around in the spaces between the droplets also absorb scattered sunlight, converting it to heat.
“Heating the cloud reduces the relative humidity in the cloud,” Jacobson said.
This causes the cloud to dissipate. And because clouds reflect sunlight, cloud dissipation causes more sunlight to transfer to the ground and seas, ultimately resulting in warmer ground and air temperatures.
Finally, Jacobson said, carbon particles released from burning biomass settle on snow and ice, contributing to further warming.
“Ice and snow are white, and reflect sunlight very effectively,” Jacobson said. “But because carbon is dark it absorbs sunlight, causing snow and ice to melt at accelerated rates. That exposes dark soil and dark seas. And again, because those surfaces are dark, they absorb even more thermal energy from the sunlight, establishing an ongoing amplification process.”
Jacobson noted that some carbon particles — specifically white and gray carbon, the variants associated with some types of ash — can exert a cooling effect because they reflect sunlight. That must be weighed against the warming qualities of the black and brown carbon particles and CO2 emissions generated by biomass combustion to derive a net effect.
Jacobson said the sum of warming caused by all anthropogenic greenhouse gases — CO2, methane, nitrous oxide, chlorofluorocarbons and some others — plus the warming caused by black and brown carbon will yield a planetary warming effect of 2 degrees Celsius over the 20-year period simulated by the computer. But light-colored particles — white and gray particles primarily — reflect sunlight and enhance cloudiness, causing more light to reflect.”
How much warming does black carbon cause?
Biomass burning was calculated to cause 20 year global warming of ~0.4 K because CAE I (~32% of BB warming), CAE II, semidirect effects, AHFs (~7% of BB warming), AMFs, and tar balls from biomass burning together outweighed indirect effects, contrary to previous biomass burning studies that did not treat CAEs, AHFs, AMFs, or brown carbon. AHFs from all sources and AMF + AHF from power plants and electricity use each accounted for statistically significant +0.03 K global surface air temperature warming.
One-dimensional simulations helped to explain the strong effect on cloud burn-off of BB and BC from other sources. They showed that when absorbing aerosols exist in clouds, instantaneous direct radiative forcing (DRF) and surface temperature change are anticorrelated because when absorbing aerosol burns off a cloud, the aerosol DRF decreases due to a decrease in optical focusing, yet surface temperature escalates rapidly due to the pouring in of sunlight to the surface. As such, particle burn-off of clouds may be a major underrecognized source of global warming.
Mark Z. Jacobson. Effects of biomass burning on climate, accounting for heat and moisture fluxes, black and brown carbon, and cloud absorption effects. Journal of Geophysical Research: Atmospheres, 2014; DOI: 10.1002/2014JD021861