The US Geological Survey has published the first-ever comprehensive estimate of carbon storage on federal lands under future climate scenarios. Initially, it looks like good news: federal lands are projected to store more carbon in 2050 than they did in 2005. However, a closer look reveals that a big chunk of these gains are dependent on the world staying on a relatively low-emissions pathway. The difference in net emissions from federal lands between high- and low-emissions climate scenarios has the potential to undercut the emission reductions expected under the Clean Power Plan. And going deeper, the study may not account for processes that could release much more carbon into the atmosphere.
What’s going to happen to the carbon stored on federal lands?
For the first time, the US Geological Survey (USGS) has published an estimate of ecosystem carbon stocks and sequestration potential of federal lands across the conterminous US. The term “carbon stocks” refers to the total amount of carbon found in living and dead components of ecosystems, including trees, shrubs, grasses, dead wood, and soils; “sequestration potential” refers to the capacity of living plants (especially trees) to take carbon dioxide out of the atmosphere and store it in their tissues for many years, through photosynthesis.
The US government owns about 23.5% of the land area in the lower 48 states (plus a large fraction of Alaska), and one goal of federal policies at the Department of Interior and the Forest Service is to increase the carbon stocks on these lands. The purpose of the USGS analysis was to evaluate how carbon stocks are projected to change as a result of future climate scenarios. These projections aimed to account for the demographic and economic impacts associated with the scenarios, in addition to the biophysical impacts of climate change. The study used an older generation of climate scenarios developed by the IPCC, comparing carbon stocks from 2005 with those estimated for scenarios A1B, A2, and B1 in 2050.
At first blush, the news seems to be good. The analysis projected an increase in carbon stocks on federal lands for all three scenarios, relative to 2005. A map figure (below) illustrates where gains and losses are expected, with maps showing estimated carbon stocks in 2005, as well as the projected changes in carbon sequestration rates by 2050, for the three climate scenarios. Most of the gains are projected to occur in the Rocky Mountain states and California, some scattered federal lands in the Southeast, and a few places in the upper Midwest and Northeast. The Pacific Northwest is expected to see significant reductions in sequestration — for reasons that are unclear — with smaller reductions across other areas, such as the Basin and Range.
The study estimated that the carbon stored on these lands would increase between 2006 and 2050 by 2153 – 2404 million tons. If we convert this additional sequestration to its carbon dioxide equivalent, it would represent 7894 – 8815 million tons CO2-equivalent (Mt CO2-e) taken out of the atmosphere. How much is that? For comparison, that’s substantially more than the 5791 Mt CO2-e emitted by the entire US economy in 2013 (the latest year for which we have verified data). So that’s great, right? Climate change unfolds, and we bank more than a year’s worth of carbon on federal lands. What’s the problem?
The global emissions trajectory really matters
As usual, the story is a little more complicated than it first appears. First, let’s take a closer look at those numbers, and then we’ll look at what’s underpinning them.
Look again at the range of carbon stocks spanning the different climate scenarios. At the low end, the higher emissions associated with the A1B scenario would result in 2153 million tons of additional carbon stock, whereas the lower emission associated with the B1 scenario would result in 2404 million tons of additional carbon stock. The difference between these scenarios, when converted to CO2-equivalents, is about 920 Mt CO2-e. That means that a higher global emissions scenario could prevent the US from storing almost a billion additional tons of CO2 emissions in federal lands by 2050. To put that in perspective, the EPA’s recently finalized Clean Power Plan (CPP) for the power sector is estimated to achieve a reduction of 870 Mt CO2-e in 2030 compared with 2005 levels. Also keep in mind that climate change would also affect private lands, which account for the remaining ¾ of the land area of the lower 48 states, so the total effect might be much larger.
Some processes that reduce carbon stocks might not be modeled well
Furthermore, we have good reasons to think that the USGS estimates of future sequestration may be too optimistic. One reason is that the researchers used a modeling approach that projects future land-use changes and changes in carbon stocks by extrapolating trends from a relatively short period, 1992-2000, and then using 2000-2005 as a base period. To estimate changes in emissions from forests due to disturbances like fire, pests, and drought, they adapted the approach taken by another USGS study, focused on Western states. Those estimates were conservative, according to the authors; in addition, they may not have fully captured the impacts of drought and pest outbreaks that accelerated in the West after 2000. Another source of bias may be the active suppression of fires that occurred during the reference period of the study – this may have dampened the model’s projections of future fires, which are expected to become much more extensive and costly.
In our own analysis of data from the Forest Service, UCS found growing impacts from fire, pests, and drought in Rocky Mountain states – the very states where USGS projected substantial increases in carbon stocks. Those three factors can trigger systemic changes in ecosystems, changing the underlying processes that affect carbon stocks. As a result, the extrapolations conducted by USGS may be based on a time period that simply doesn’t represent the dynamics of the future. Cascading impacts of drought, pests, and fires may put the western forests on a much different trajectory, potentially leading to higher emissions and lower carbon sequestration. Other regions will face their own complex climate impacts, and federal lands in the Southeast, upper Midwest, and Pacific Northwest may also fall short of the study’s projections.
The USGS study also may not have accounted for the turnover of forest species that is expected with climate change. If we look more closely at the Rocky Mountains, for example, we see that the future climate may become unsuitable for several of the most common tree species found there today, including lodgepole pine, ponderosa pine, and Douglas fir. If so, those species may be in decline by 2050. It’s unclear what new species, if any, would move into those areas to replace them. Areas that USGS projected to see sizeable increases in carbon stocks – such as western Montana and central Colorado – are going to be less suitable for the species that currently dominate those forests. Can carbon stocks really increase if the dominant species are in decline?
The changes in climate that drive these declines may be manifested in tree-killing droughts, pest outbreaks, extensive wildfires, or some unforeseen combination of these impacts. The wildfires of 2015 have exceeded our expectations in their number, size, and ferocity – with veteran firefighters reporting that this year has been unlike anything they’ve ever seen. If this year becomes the norm, we may be entering a much different era of climate-driven changes in our western landscapes.
What should we make of all this?
Perhaps the most useful pieces of information to take from the USGS study are the projections related to the more optimistic climate scenarios. The study suggests that if the world coordinates its efforts to reduce heat-trapping emissions and if the US takes active steps to anticipate the impacts of climate change on federal lands, we might preserve and even enhance our vital carbon stocks. These two actions need to happen in tandem. Our actions in the energy sector could actually lead to a windfall benefit in the land sector, if we manage it successfully – a windfall that might be comparable to what we expect to achieve through energy sector reform under the Clean Power Plan. And there’s an extra benefit: our children and grandchildren might have the chance to appreciate the same kind of the beauty and value we currently enjoy on our public lands today. That would be a huge win for everyone.
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