Buying Time on Climate Change

While much of the discussion of climate change revolves around carbon dioxide because of its primary role in increasing average global temperatures, many other chemicals other than CO₂ can have an effect on the climate. Two such important contributors to global climate are the greenhouse gas methane (i.e. the primary component of natural gas) and black carbon (soot from incomplete burning). Methane is emitted by a variety of sources both natural and human such as wetlands and rice paddies, as I had discussed in a previous article about the methane cycle. Black Carbon is nothing more than the smoke common in wood fires or any kind of incomplete burning from diesel or coal burning. It also can absorb heat in the atmosphere and cause warming, and when deposited on snow or ice, can decrease its cooling ability by reducing its albedo or how much light it reflects. Both methane and soot have much shorter lifetimes than carbon dioxide in the atmosphere, at 11 years and weeks to months respectively compared to 170 years for carbon dioxide, making them attractive targets for short to medium term reductions in emissions related to climate change.

In addition to their effects on climate, methane and black carbon are also problematic emissions because they both act as precursors to photochemical smog and because black carbon is a big component of particulate emissions that are really bad for your lungs. Because reducing both these emissions would have a public health benefit beyond any changes to the climate, emission reductions of methane and black carbon could potentially be an important first stem to slowing climate change. In a recent Science article (gated), Drew Shindell, a NASA scientist, and a team of scientists and economists from around the world identified various technologies available now and possible regulations that could reduce methane and black carbon emissions immediately. They then calculating the costs of of implementing these new technologies and compared them to the potential benefits on public health, crop yields, and climate. The benefits of reducing both far outweighed the costs, in reduced impacts from changes in climate from differences in precipitation patterns, crop yields, and premature deaths. In particular, the reductions in black carbon resulted in an enormous benefit in terms of reduced premature deaths since particulate emissions such as black carbon are pretty bad for human health. Most of the benefits were realized in India and China (where black carbon emissions are much, much worse than in the United Stats or Europe) in the form of avoided changes in the precipitation patterns crucial for agriculture. However, the United States also benefited from potential increases in crop yields due to less photochemical smog from methane emissions. The ozone formed in photochemical smog, in addition to being a lung irritant, also reduces crop yields. Plants reduce atmospheric uptake in high concentrations of ozone because of its highly reactive nature, which can damage the inside of the plant as easily as it can damage your lung tissue.

For climate change, the full implementation of their recommended technologies and regulations would actually in the short to medium term help slow the pace of climate change relative to a "business as usual" scenario. This interactive graphic from NASA that accompanied this publication shows how under different "climate sensitivity" (how much the temperature changes in response to increased CO₂) scenarios the implementation of CH₄ (methane) and BC (black carbon) measures can reduce the expected warming. Depending on how sensitive the climate is to changes in CO₂ these measures delay the time it takes to reach 2^oC degree warming threshold that many scientists consider to be a dangerous threshold for the climate. This gives all of us some "breathing room" (so to speak) to develop alternatives to fossil fuel burning as a source of energy because even with these measures CO₂ is still the primary mover of climate change. Many times it's extremely under-appreciated just how challenging it is to develop alternatives to fossil fuels as it may be decades before a true alternative can be developed. This study gives us hope that we can eventually tackle climate change without dramatic changes to living standards or a halt to the increasing prosperity that free trade and liberalizing reforms are bringing to the developing world.

Related Links:
An interview with Drew Shindell, the primary author of this study

Nuclear Bomb Testing and Boreal Forest Fires

One important piece of evidence that shows that climate change has been caused by humans is the decrease in the amount of radiocarbon (carbon-14) in CO₂. You may be familiar with carbon-14 because of its use in radiocarbon dating in archaeology for objects up to 60,000 years old. For those unfamiliar, though, carbon-14 is produced in the upper atmosphere by reaction of neutrons from cosmic rays with nitrogen. The carbon-14 produced is then eventually transported to the lower atmosphere, where plants uptake the ¹⁴CO₂ via photosynthesis. Once a plant dies, it stops absorbing carbon dioxide, which then causes any of the carbon-14 in the leftover plant material to radioactively decay. As such, fossil fuels, having been in the ground for millions of years, contain almost no carbon-14, so that when they are burned, the carbon dioxide produced has no carbon-14 content.

Of course, measuring carbon-14 in CO₂ in the atmosphere has several complications associated with it. In an article in the Journal of Geophysical Research, a group of scientists from NASA, NOAA, and various universities around the world measured CO₂ and its carbon-14 content from air samples collected on aircraft campaigns over the Arctic in Canada. As you might expect from my above explanation, CO₂ from right over the Alberta tar sands has both higher concentrations and reduced carbon-14 content compared to typical levels for background CO₂ from emissions from burning the oil extracted. Interestingly, some of the air samples that were far from any influence from civilization also had higher concentrations of CO₂ but increased carbon-14 content. High levels of carbon monoxide in these samples along with some atmospheric modeling (and a little knowledge of local conditions) led the scientists to conclude that these measurements were sampling air from boreal forest fires in the Arctic. Why would this lead to higher carbon-14, though?

Click for more information from Wikipedia. Image from public domain

The above plot with the amount of carbon-14 in the atmosphere over the last few decades helps show what likely caused this increase in the carbon-14 content. During the 1950's and into the 1960's, the world's governments conducted many nuclear bomb tests in the atmosphere, which released a lot of neutrons into the atmosphere. Since neutrons cause the formation of carbon-14 from nitrogen as explained above, this nuclear testing caused a so-called "bomb spike" in carbon-14 until atmospheric nuclear testing was banned internationally in the 60's from health concerns about the release of other much more radioactive isotopes of elements such as strontium-90 into the atmosphere. Because plants continuously absorb CO₂ throughout their lifetimes, decades-old trees in boreal forests in the Arctic could retain high levels of carbon-14 until burned by a sudden forest fire, releasing highly enriched carbon-14 back into the atmosphere.

The results in this paper highlight some of the challenges involved in the use of carbon-14 as a tracer of fossil fuel burning. However, it also shows the incredible utility of trace isotope studies in providing information about the chemical processes in the atmosphere. Using isotopes, much more information can be gained about a chemical than from simply looking at the amount present alone.