Cooling effect of preindustrial fires on climate underestimated

From Cornell Chronicle

By David Nutt |

October 1, 2018

Atkinson Center for a Sustainable Future

This graphic shows the decline of black carbon emissions from fire activity from 1700 to the present, contrasted with the rise in global population.

The Industrial Revolution brought about many things: the steam engine, the factory system, mass production.

But not, apparently, more wildfires. Actually, the opposite.

A new study, “Reassessment of Pre-Industrial Fire Emissions Strongly Affects Anthropogenic Aerosol Forcing,” by a Cornell postdoctoral researcher, published in August in Nature Communications, finds that emissions from fire activity were significantly greater in the preindustrial era, which began around 1750, than previously thought. As a result, scientists have underestimated the cooling effect the aerosol particles produced by these fires had on the past climate.

As fire burns, tiny particles – aerosols – are released into the atmosphere, where they can increase the brightness of clouds and reflect sunlight back into space, cooling the planet in the process (also known as indirect radiative forcing). This cooling can help offset increased warming caused by anthropogenic greenhouse gases like carbon dioxide.

“Most people are probably very familiar with the idea of greenhouse gas warming but are less aware that human activities can also create a cooling at the same time, through changes to cloud properties via emissions of aerosols and their pre-cursor gases,” said lead author Douglas Hamilton, postdoctoral researcher in earth and atmospheric sciences. “You don’t see the full impact of the warming from the greenhouse gases at any point because you also have these aerosols. It’s really important for us to understand the cooling effect from these aerosols in order to understand the overall impact human activity has on climate.”

To gain a clearer picture of aerosols’ historical impact, Hamilton examined fire proxy records, such as ice cores, that hold black carbon emitted from preindustrial fires; charcoal depositions in lake and marine sediments; and scarring in tree rings, along with present-day satellite data documenting the decline in the burnt area caused by fires in recent decades. These paleoenvironmental archives show that fire occurrences worldwide peaked around 1850 and fire emissions have dropped between 45 to 70 percent globally since the Industrial Revolution.

While common sense might suggest fires would rise as human density increased around the planet, in actuality, the establishment of cities, fire departments and local infrastructure, plus the reduction of forests for agricultural purposes, have all curtailed the spread of wildfires, Hamilton said.

Climate change and land-management practices, however, may be reversing that trend. Recent years have seen an increase in the amount of fires in the U.S., for example.

“In some regions we’re now starting to see an increase in the amount of fires, and it’s projected to continue,” Hamilton said. “But where the fires are and where they will increase in the future is not the same as where they were in the past.”

The paper concludes that preindustrial fire emissions are the single largest source of uncertainty when it comes to understanding the magnitude of climate warming caused by manmade forms of combustion.

Black carbon: friend or foe?

That sense of uncertainty around aerosol impacts on the climate also informs a separate paper Hamilton recently coauthored, “Black Carbon Radiative Effects Highly Sensitive to Emitted Particle Size When Resolving Mixing-State Diversity,” also published in Nature Communications in August. That study – led by Hitoshi Matsui, a former visiting scholar at Cornell and now at Nagoya University in Japan – finds that better measurements of the size of black carbon particles, and the ways these particles mix with other aerosol compositions in climate models, is more important than previously thought to understanding black carbon’s heating effect in the present day, and how it could change in a future with potentially more wildfires and less fossil-fuel burning.

Black carbon is formed by incomplete combustion of fossil fuels, biofuels and wildfires. Because of its dark color, it absorbs sunlight and warms the planet. The strength of this warming is determined by a particle’s size and how diluted it is by other aerosols – such as clearer, organic carbon – or by the condensation of gases that then mix with it.

The researchers developed a more detailed model of black carbon than is currently used. The model factors in a wide range of particle sizes and the different ways black carbon can mix with other atmospheric constituents to show just how nuanced these atmospheric interactions can be. Understanding these interactions is particularly important because one proposed way of mitigating the human impact on the climate is actively reducing only black carbon aerosols while not eliminating others.

“Properly describing the particle size of black carbon particles and their mixing with other aerosol components is very important to understand the contribution of black carbon to the current climate and its future changes,” Matsui said.

“What we’re showing here in this new advanced model is that, as fires increase in the future, the additional warming that was predicted in more basic models could be an actual cooling relative to present day, because we resolve the size and composition of black carbon in more detail, combined with what is going on with other aerosol and gases that are also co-emitted with the fires,” Hamilton said.

Both of these studies add nuances to how effective reducing black carbon to improve air quality and reduce climate change will be, according to Natalie Mahowald, the Irving Porter Church Professor of Engineering and Atkinson Center for a Sustainable Future faculty director for the environment, who coauthored the particle-size paper.

“We really need to understand more about preindustrial fires and how we’re changing the size distribution of the black carbon emissions. That’s the bottom line,” Mahowald said. “As we try to move forward and solve problems with air quality and the climate, we need answers to these questions.”

Both studies received financial support from the Atkinson Center.


36 thoughts on “Cooling effect of preindustrial fires on climate underestimated

  1. “In some regions we’re now starting to see an increase in the amount of fires, and it’s projected to continue,”

    This is only true because of arson if it is true.

    • In California the frequency and severity of wildfires is exacerbated by forest management practices forced on us by the environmentalists.

      We have been suppressing forest fires for a long time. It used to be a low tech endeavor. The tools were axes and shovels, pumps and water bombers weren’t available. link That means forest fire suppression is available even in underdeveloped nations.

      Humans have a great deal of control over wildfires and forest fires. It is not obvious that CAGW per se would affect their frequency and severity.

      • Controlling fire by small burns and fuel removal is no tech stuff. The Indians did it without metal. We could do it too.

        We will not live well or happily until the last lawyer is strangled with the entrails of the last environmentalist.

    • Careful, they’ll start hooking up air masks with diesel exaust again in order determine mortality rates on human test subjects by particulates.

  2. What we’re showing here in this new advanced model is that, as fires increase in the future, the additional warming that was predicted in more basic models could be an actual cooling relative to present day, because we resolve the size and composition of black carbon in more detail, combined with what is going on with other aerosol and gases that are also co-emitted with the fires, …

    In other words, soot from forest fires cancels out the warming effect of CO2. That leads to the conclusion that the modern warming is due to a decrease in forest fires, not an increase in CO2.

    The other thing that occurs to me is that soot is a result of incomplete combustion. In other words, soot indicates that some CO2 wasn’t created by a fire. Could it also be that the modern increase in CO2 is also a result of fewer forest fires. If wood is allowed to rot rather than burn, it should generate more CO2.

    The findings of these studies could be earth shattering. Global warming is due to fewer forest fires! That would shake things up.

  3. norton anti virus is still telling me it is blocking attacks when I log into a Watts Up article… redirection to some scam website… just FYI, mods… not happening on the rest of the web for me…

    • Griff: “norton anti virus is still telling me it is blocking attacks”

      I also have norton anti virus on my PCs. I don’t get a pop-up message from norton showing a problem with WUWT but it may be just logging the problem and blocking the site. Can you tell me what norton log file the message is recorded in and what the text of the message is? I’d be interested to see if I am getting a similar message and help confirm the problem.

        • Griff:

          If the message is from Norton anti-virus, besides the pop-up, a similar message should also appear in one of the the Norton anti-virus software’s log files (probably the Traffic Log, but possibly in one of the other logs). If you can’t find it in any of the log files, the message is probably coming from somewhere other than Norton Anti-virus.

  4. Near the top of the article we are told that fires produce aerosols that reflect sunlight thus cooling the planet. Further down we are told that black carbon released in the same way absorbs heat and warms the planet. Now, not all aerosols released by fires are black carbon and indeed not all black carbon comes from wildfires. However, where is the analysis of the relationship and scale of these two opposing factors?
    Which is more important over time – carbon heating or cooling as a result of wildfires? Is that not the fundamental issue at stake?

    • The same is true of water vapor. It is a strong greenhouse gas, but in the form of clouds, a strong reflector.

  5. So now we know why Global Warming is actually causing Global Cooling.

    So its s something like this, CO2 causes heating, but it also causes more growth, i.e. Greening. That means that we have a lot of potential fuel waiting for something, lightning or humans to start it burning .

    But wait, here is the good news, that burning not only causes even more CO2, which helps Green even more plants, but it also causes Cooling.

    So what is the problem, it all balances things very nicely, problem solved.

    The Greenies will just have to find another “Cause” to protest about.


  6. Also it would be interesting to understand how increasing pollution in China and reduced pollution in the west has affected weather patterns. One of my hobby horses for a long time.

  7. I have seen aerial photos taken of my locale in the beginning of the C20th and the thing that stands out is the lack of trees. I presume this is down to the fact that they were a ready resource that everyone tapped into in order to keep warm in winter. People would have been burning them on open fires but the smoke from a small fire would not rise as high in the sky as that from a major wildfire and of course wildfires would’ve been less likely because of the reduced fuel load.

  8. There is still much work to be done impact of human activity on forcing in preindustrial era.

    Burning obviously introduces CO2 into the atmosphere. To the extent it clears forested areas it also diminishes the aerosol effects of forests and changes albedo.

    Human effects are not limited to burning biomass but also include methane production from livestock and rice farming in Asia.

    It makes sense there would be more preindustrial fire activity than industrial because by the industrial era major portions of arable land had already been cleared.

  9. Black carbon is formed by incomplete combustion of fossil fuels, biofuels and wildfires. Because of its dark color, it absorbs sunlight and warms the planet. The strength of this warming is determined by a particle’s size and how diluted it is by other aerosols – such as clearer, organic carbon – or by the condensation of gases that then mix with it.

    No. It will warm the atmosphere.
    As the atmosphere is on average colder (at minus 15 degC) than ‘The Planet (at plus 15 degC, subject to regular revision & adjustment of course), the atmosphere CAN NOT radiatively warm (the surface of) The Planet.
    Stefan Boltzmann explicitly stated as much

    As fire burns, tiny particles – aerosols – are released into the atmosphere, where they can increase the brightness of clouds and reflect sunlight back into space,

    Wait. I thought we’ve just been told these things absorb sunlight – not reflect it.

    cooling the planet in the process (also known as indirect radiative forcing).

    What sort of garbage statement is ‘indirect radiative forcing’

    Please tell me this is a Bad Dream and I’m not actually hearing this.
    You cannot cool *anything* with *any* sort of radiative forcing, direct or indirect.
    Even Cap’n James T Kirk or even Picard didn’t have that kind of techo-garbage toy.

    As a First Lesson in Green House Gas theory, go back to school and *TRY* to get your head around how electromagnetic energy moves through a vacuum.
    A place, by definition, of Total Nothingness

    Why is the speed of that radiation’s propagation limited to what it is (300 million metres per second)
    What is in that empty space to hold it back.

    What puts the brakes on, why doesn’t it move at infinite speed?
    Why does a place of complete nothingness have measurable ‘properties’?

    And me or you can measure those properties on our own kitchen tables using a coil of wire, 2 pieces of tin-foil, a voltage source and a voltage meter.
    We can each calculate the Speed of Light from experiments we can do in our own homes- no need for stop-watches and the shooting of lasers at mirrors across the length of football fields.

    You DO understand that Radiation is NOT a variation on (thermal) conduction?

    Until a few more people get that through their heads, we will continue to see self-contradictory and absurd sciencey sounding garbage like this. And the Theory of the GHGE

    Notta lotta hope though is there.
    British Universities are now dragging people off the street to fill places on their courses. People who have, over the course of 17 or 18 years, demonstrated no skill or desire to absorb or learn any sort of knowledge.
    I’ve met some of them and they are actually proud of how dumb they are – failing all your schools exams gains you a Badge of Honour.

    Methinks that has been going on for quite some time- certainly explains current UK politics not least

    • Peta,
      Nice rant!
      How about you write “a WUWT idiot’s guide to atmospheric physics as relevant to climate science” (or similar) explaining the points you have made above in a little more detail? Once published, you could then show a link every time you comment on such papers in future.
      As you will have gathered from my own comment on this odd paper above, I’m not disagreeing with you for a second, but I’d love to learn more about this subject which is clearly so crucial to climate science and AGW.

  10. This discussion reveals something — how complex the climate is and how much we underestimate it.

    Just one element of the climate — wildfires, has so many major and minor contributing factors, each of which affect the others in a recursive manner, and no one fully understands it. Warming effects vs cooling effects, black soot vs. contrails, reflection vs. absorption vs. insulation, biomass growth vs land use vs. management practices, air pollution particle sizes, height that fires reach, etc., etc. And even now they keep discovering new factors as well.

    … and yet this is just one small element of the overall climate system;

    … and yet they claim to have enough understanding of this climate system, and enough good data, to project ‘with high confidence’ what will happen 100 years from now.

    Is that foolishness, arrogance, or just vanity?

  11. “Here we show that revised assumptions about pre-industrial fire activity result in significantly increased aerosol concentrations in the pre-industrial atmosphere. Revised global model simulations predict a 35% reduction in the calculated global mean cloud albedo forcing over the Industrial Era (1750–2000 CE)”

    A) Develop and run a model that redefines history to better suit their confirmation bias.

    “When compared to 26 other uncertain parameters or inputs in our model, pre-industrial fire emissions are by far the single largest source of uncertainty in pre-industrial aerosol concentrations, and hence in our understanding of the magnitude of the historical radiative forcing due to anthropogenic aerosol emissions”

    B) They build their analysis upon unknowns and assumptions.

    “Globally, most fire ignitions are caused by humans, which makes a positive scaling of total burned area, and hence total fire emissions, with human population density logical at first”

    C) Introduce their ‘confirmation bias’ belief that fire is man’s fault and almost solely man’s fault.

    “A large number of global fire models exist, of which six contributed to the creation of historical fire emissions in the CMIP6 dataset. However, most fire models still assume an overall positive relationship between human population and fire”

    D) Claim that the existing datasets, which are used as the primary inputs to the author’s new model, do not blame mankind sufficiently.

    “Supplementary Table 1. The PD/PI change in black carbon (BC) and specific fire trace gases
    (Vanillic Acid (VA) at D4 and Levoglucosan (LG) at NEEM) concentrations with standard error
    of the mean (SEM) at two Greenland ice core sites (D4 and NEEM) calculated using different
    averaging time periods. NEEM value in parentheses includes a recent extreme high

    Supplementary Table 2: Fire aerosol and gas emissions for each of the four main PI
    experiment datasets, four PI climate sensitivity scenarios and single PD dataset used in the
    calculation of the aerosol radiative forcing between the PI and PD. The different
    representations of anthropogenic land use and modelled land cover behind each scenario are

    Supplementary Table 3. Global and regional pre-industrial burnt area estimates. Regions
    defined as: Northern Hemisphere (NH), Southern Hemisphere (SH), United States (US),
    Europe (EU) and Australia (AUS).

    Supplementary Figure 1. Greenland ice core fire proxy concentrations.”

    E) 1) The timeframe for their “preindustrial” assumptions if 1700 to 2000 CE.

    E) 2) These reconstructions and modele results are used as the inputs for the author’s model.

    “We compare modelled PD/PI BC atmospheric concentration ratios (see Methods, Supplementary Figure 5) to measured BC ice core ratios at four ice core locations: two in Greenland, and one in each of North America and Europe (Fig. 2). To minimise the large uncertainties associated with the absolute deposition, we analysed only the ratio of PI to PD BC in the cores, assuming that trends can be more accurately modelled than absolute values”

    F) Their four ice core reconstructions are run through another confirmation bias assumption filter.

    “The managed burning routine of the fire module defines that 50% of the litter on 20% of the used land (cropland and pasture) is burned annually. As the seasonality of burning on used land is not well known at global scale, fire emissions are partitioned evenly across all snow-free days of the year with mean temperature above 0 °C. In reality, most biomass burning on used land probably occurred during certain periods of the year, e.g., after harvest, but it is currently beyond the capability of any global land use scenario to prescribe these periods.

    To perform a PD simulation with LMfire, we made a transient model simulation for the period 1701–2000 driven by the synthetic climate timeseries described above, transient land use, and reconstructed and measured atmospheric CO2 concentrations.”

    G) Not to overlook that the author’s used a daily fire and black carbon emission in their their model; even as they recognized that assumption was not valid.

    Another infinite self gratification model where author assumptions override logic and history while creating a new CO₂ and black carbon imaginary hypothesis.

  12. In the early 1960s and 1970s, one sought a career or publicity by explaining “Global Cooling”.
    Rasool was one such researcher and he went on about particulate carbon in the atmosphere causing cooling.
    There was a newspaper article published in 1971. More recently,there has been comment that James Hansen provided the “modeling” for such a theory.
    It would be interesting if there was any evidence that Hansen then, himself, was pushing the story about particulate carbon and cooling.
    Because later on, he was strongly touting that it then “caused” global warming.

  13. “These paleoenvironmental archives show that fire occurrences worldwide peaked around 1850”
    If aerosols were a controlling factor as claimed (in a time before significant human CO2 emissions), the period between 1750 and 1850 would have been marked by cooling. Was it? if not, the speculation that:

    “the cooling effect from these aerosols” overpowered other natural factors (whatever caused warming which ended LIA) is disproved.


  14. Brings to mind that when colonists came to what would become eastern USA they encountered terrain that Indians had cleared, to varying degrees, by managed fire. The hunting grounds were not impenetrable in part due to that. I do not know what time frame was involved or if any
    temperature reconstruction would show a cooling effect.

  15. In the game of Climate Poker, aerosols, from fires and volcanoes are often the Carbon Cultists’ wild cards, which allow them to win every time, or to claim they do. But this climate “researcher” appears to have made an own-goal, by claiming that wildfires, by virtue of being greater back in 1750 were cooling the climate. That means that without the wildfires, it would have been warmer. But that means the actual “warming effect” of CO2 would be that much less. Oops.

  16. So more fires kept us cooler.

    Funny how temperatures rise during WWI and WWII. Lot of fires then, but also a lot more aviation.

    Living in an area where as many as 200 high flying jets fly by daily with contrails creating high cirrus clouds I wonder if much of the warming thats taken place over the last 60 years is not due to increased aviation. This would increase night time temperatures and hence tbe mean while day time temperatures over land would see little increase, something the latest US study seemed to show.

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