Sinks & Sources: The Role of Forests in Carbon Sequestration…and Why it Matters Werner A. Kurz, PhD

Dr. Kurz is a Senior Research Scientist at the Canadian Forest Service (Natural Resources Canada) in Victoria, British Columbia. He leads the development of Canada’s National Forest Carbon Monitoring, Accounting and Reporting Systems. His research focuses on the impacts of natural disturbances, forest management and land-use change on forest carbon budgets. Dr. Kurz has co-authored five reports of the Intergovernmental Panel on Climate Change (IPCC). He serves as adjunct professor at both the University of British Columbia and Simon Fraser University. He received a Ph.D. in forest ecology from the University of British Columbia in 1989, and an honorary doctorate from the Swedish University of Agricultural Sciences in 2009.

Let me start with some basics: Fifty percent of the dry weight of wood is carbon. One cubic meter of wood—that’s about the size of a telephone pole—contains about a quarter ton of carbon, or the equivalent of about a ton of CO2 (carbon dioxide). Figure 1 is a picture of a million cubic meters of wood. It’s a salvage logging operation in Sweden after a massive wind-throw event. The intent was to clear the space for new trees to grow and to use some of that wood for purposes other than just decay. The wood pictured here contains about a quarter million tons of carbon. Humans are burning fossil fuels that emit 8 billion tons of carbon into the atmosphere every year. In terms of carbon, that amount is equal to half of the trees in Canada’s forests … this picture times thirty-two thousand. That’s enough wood to make a two-by-four that you can wrap around the earth at the equator. Not once or twice, but over 200,000 times.

The reason I’m telling you how much carbon we emit into the atmosphere is because it’s a precursor to understanding why CO2 concentrations are increasing. This is the so-called Keeling curve, shown here in Figure 2, that the late Charles Keeling developed in 1958 to measure the CO2 concentration on the northern hemisphere of a volcano in Hawaii. What we see is that CO2 concentrations in the earth’s atmosphere have been steadily increasing. We have, in fact, recently surpassed [an atmospheric CO2 concentration of] 391 parts per million, which is 40 percent above pre-industrial age CO2 concentration levels of 280 parts per million that have been reconstructed from ice cores.

What is more concerning is the rate of increase in our atmosphere, which in the 1990s was about 3.2 billion tons of carbon per year. Since then, we have had the Kyoto Protocol, the United Nations Framework Convention on Climate

Figure 1: One million cubic meters of wood salvage logged after a major windstorm in Sweden. The carbon content of this wood pile is about one quarter million tons of C. Annual carbon released globally from burning of fossil fuels (8 billion tons of C and growing) is equal to about 32 thousand times the amount of C contained in this wood pile. Photo credits: Ole Nilsson, Sweden.

Figure 2: Northern hemisphere atmospheric carbon dioxide concentration as measured at Mauna Loa, Hawaii, Data source: http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mlo.html.

Figure 3: Annual cycle of carbon dioxide concentration changes in the northern hemisphere shows C sink during the summer months and C source during the winter. Data source: http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mlo.html.

Change, and numerous commitments to reduce greenhouse gas emissions into the atmosphere. In the period 2000 to 2009, the rate of increase has risen to 4.1 billion tons of carbon per year. In other words, we’re heading in the wrong direction at an accelerating pace. If we take a closer look at this curve in Figure 2, we see that the mean is increasing over time, but the monthly values are showing a very distinct seasonal pattern in the northern hemisphere.

If you want, we can observe the breathing of the earth. In spring, when leaves come out and plant vegetation in the northern hemisphere starts photosynthesis, the uptake of carbon dioxide from the atmosphere exceeds the releases; and we say the forests and the landscapes are operating as sinks. Then in fall and winter, when there is less or no photosynthesis and emissions from decomposition and decay exceed the uptake from photosynthesis, these systems operate as a source.

Although this is a schematic (see Figure 3), it is deliberate that the uptake arrow is greater than the release arrow because, if we look in detail at human perturbation in the global carbon cycle, we see a very interesting phenomenon. In the period of 2000 to 2009, we have been adding an average of about 7.7 billion tons of fossil fuel carbon to the atmosphere and another 1.1 billion tons from land-use change, which is from deforestation [mostly] in the tropics, the conversion of forest to non-forest land uses. The interesting thing to note is that, of this 8.8 billion tons of carbon, only half remained in the atmosphere. In other words, we humans have been getting a 50 percent discount in terms of the atmospheric CO2 concentration increases for our emissions. For every ton of carbon that we emit, half a ton is taken up by biological systems, mainly oceans and terrestrial systems around the world.

The big question is whether this 50 percent discount will continue in the future. In other words, if climate change and the things we are talking about today are detrimentally affecting the future function of forests as carbon sinks, then the rate of increase of CO2 in the atmosphere will go up greatly. Do this thought experiment: take away the sink of 2.4 billion tons of carbon that forests are contributing, and you will immediately see that the equation can only be balanced by having the rate of CO2 increase in the atmosphere go up. Worse, imagine if these forests were to become sources on a large scale — the rate of CO2 increase in the atmosphere would go up even more.

Central to the scientific questions we’re struggling with right now is this: What will be the response of forests, and terrestrial ecosystems more broadly, to changes in climate? Pierre Friedlingstein and colleagues did a comparison of a number of different models of how terrestrial ecosystems might respond and found that there’s a lot of scientific uncertainty; but, qualitatively, we can observe two possible responses. One is that, in a warmer world, trees grow better and there is more uptake of carbon from the atmosphere, what scientists call “negative feedback.” The other is that, with more warming, we’ll get accelerated decomposition, thawing of permafrost, release of carbon from peat and tundra systems, more forests fires, more insect attacks – what scientists call “positive feedback.” In other words, warming will feed the warming.

Figure 4: The impact of the Mountain Pine Beetle outbreak on forest carbon stock changes in British Columbia, Canada estimated from analyses based on model simulations with and without beetle impacts. Source: Kurz et al. 2008.

Now this isn’t just a scientific uncertainty, because the world is struggling to achieve a stabilization of CO2 concentrations in the atmosphere. The uncertainty among the models by 2100, the end of the century, is about 16 billion tons of carbon per year in fluxes; that’s twice the rate of current carbon emissions from fossil fuel burning. When we want to achieve a certain stabilization target, our mitigation effort will depend greatly on whether or not terrestrial ecosystems will help or hinder. In a world where the out-gassing of CO2 from terrestrial ecosystems exceeds the emissions from fossil fuel, we can reduce our fossil fuel emissions by a great amount and CO2 concentrations will still continue to rise.

We have already heard a bit about the mountain pine beetle, and I want to add just a few words about the impact of the beetle on the carbon balance in British Columbia. I know you have a problem here in the [U.S.] West. In British Columbia, the scale of the outbreak is much greater. We had extensive forest fires in the period 1880 to 1920 followed by natural regeneration of predominantly pines or pine-mixed forest. In addition, the successful protection against fires, combined with low harvest rates, has resulted in a landscape with large numbers of old pine trees. More recently, we have had warmer winters that allow higher overwintering survival rates [of mountain pine beetles], and warmer summers that allow higher reproductive success of the beetles, which have expanded their range northward and into higher elevations.

The area now affected in British Columbia is about 41 million acres, an area the size of Wisconsin. When a beetle kills a tree, it has two implications for the carbon balance. First, the tree is no longer removing carbon dioxide from the atmosphere. So, the rate of uptake is decreased, and it starts to decompose. So not only are we reducing the uptake, we are also increasing the release of CO2 into the atmosphere. In 2008, we published a paper in the journal Nature where we quantified the impact of that outbreak in British Columbia. The blue line that you see here on this graph in Figure 4, shows the annual carbon balance of these forests if there were no beetles in the landscape. In the model, when we conveniently turn off the beetle, you can see that these forests are predominantly a small carbon sink, except in 2003 and 2004 when we had big forest fires in the area. Assuming average forest fire conditions in the future, which one can argue about, we find that, once we turn the beetle back on, the carbon balance in these forests switches from being a sink to being a substantial source.

Over the 20-year outbreak, the beetle impact is about a billion tons of CO2 added to the atmosphere. The annual values currently – we happen to be the peak of the outbreak – are about 73 million tons of CO2 per year. To put that in perspective, the CO2 emissions from all other sectors in British Columbia — burning fossil fuel, coal, natural gas, you name it — are about 65 million tons per year. So the beetle impact is greater than the CO2 emissions from all other sources in British Columbia, which, admittedly, is a large province with a small population density.

Understanding the direction and magnitude of feedback to climate change is essential. Climate changes will affect many processes, including growth, decay and disturbance, with large differences between ecosystems and regions around the Northern Hemisphere and the world. We’re currently not really able to predict the net impacts of all these changing processes together. However, what I dare to say is that there is an asymmetry of risk. As somebody said earlier, it takes a lifetime to grow a tree; it takes one bad event to kill the tree. An analysis that we have done for the boreal forest [of Canada] indicates that it is very unlikely that productivity increases in the forest can offset the increased losses associated with the changes in disturbance regimes. That is supported by everything we’ve heard and will continue to hear today. We need to have more monitoring and more modeling efforts to quantify and understand the magnitude and the direction of the feedback [to climate change].

One of the messages I want to leave you with is that forest and, more broadly, terrestrial ecosystem responses to climate change have the potential to provide positive feedback to future climate change through increased emissions that could completely negate the benefits of our mitigation efforts in all other sectors. In other words, if we let climate change advance too far, then terrestrial ecosystems could start to pump out vast quantities of carbon into the atmosphere. For example, the circumpolar tundra contains three times as much carbon in soil and peat lands than is in the atmosphere. If these systems start to respond in a massive way to climate change, human efforts to reduce fossil fuel emissions could be completely swamped.

I showed at the beginning of my talk that we have been receiving a 50 percent discount on all fossil fuel emissions, and about 27 percent of that can be attributed to the sinks in forests. Limiting climate change is the first important step towards helping us maintain the forest carbon sink. As I showed, climate change impacts on forests could increase atmospheric CO2 concentrations by triggering changes in processes – for example, widespread mortality, accelerated decomposition rates and more fires – that could substantially increase the atmospheric CO2 concentrations.

Lastly and very briefly, I think that what this all leads to is the question of what we’re going to do about it. We don’t have time to go into depth, but I do want to highlight that sustainable forest management and the use of wood and wood products as substitute for more emissions-intensive material, such as concrete and steel, can contribute to climate change mitigation efforts. When a ton of concrete is produced, one ton of CO2 emissions is created. And you can’t get very far with a ton of concrete in terms of building something. So understanding how we can use dead trees created by climate change, how we can use the opportunities created by climate change to grow new trees in regions where they may not have grown before, or where we can enhance productivity through other forest management options will be important to helping us de-carbonize the atmosphere. Forests and forest management will continue to play a role in the future in influencing the carbon cycle, but by themselves, they cannot overcome the problem of fossil fuel emissions.