It’s hard to overstate the importance of understanding how carbon moves through land, oceans, air, and all life on Earth. The carbon cycle has helped shape the planet’s climate back into deep time and is a primary determinant today. It is then not welcome news that we may be faced with losing some of our capacity to measure carbon dioxide and other greenhouse gases under looming budget cuts.
A group of prominent scientists in this field recently described their concerns and scientific impacts in a letter to Science. So why is understanding the carbon cycle actually important for us?
Carbon cycle at your service
Over the past 150 years or so a lot of carbon dioxide has been dumped into the atmosphere through human activities, such as burning fossil fuels and deforestation. This carbon dioxide is very effective at trapping heat and changing our climate.
The carbon cycle reduces the impact of these emissions; natural sinks for carbon dioxide remove over half of what we put into the air. Roughly half of this service is provided by land vegetation (plants take up carbon dioxide during photosynthesis to produce stems, foliage, and roots) and the other half by ocean uptake. So, for every billion tons of carbon emitted to the atmosphere, where it is a potent heat-trapping gas, less than half remains there. Land and oceans take away the rest.
This climate service does not come without a cost, though, as the oceans are increasingly becoming more acidic as they remove some of our carbon emissions. The carbon cycle is also very complex. Some of the carbon emitted into the atmosphere remains for hundreds to hundreds of thousands of years until it is finally removed by very slow processes such as deep ocean mixing and rock weathering. This gives rise to the concept of “irreversible” climate change. So the carbon cycle helps us in some ways, but is very nasty in others.
Shooting wobbly arrows at a climate target
Putting aside the discussion of whether climate targets (i.e. how much warming we think we can live with) are a good idea and at what level they should be, let’s assume there is some level of warming we don’t want to exceed. In international climate negotiations this has been a global increase of 2°C (3.8°F).
It has been shown that there’s a “simple” relationship between total (or cumulative) carbon emissions and long-term temperature increase. A portion of what we emitted a hundred years ago is still having an effect, so we have to look at emissions since the Industrial Period began. So essentially we have a bank account of carbon we can spend (or emit) if we were to stay below the internationally proposed temperature goal. The “simple” is in quotes, because there are uncertainties about how much we have in our account.
To hit a target we might have more in our account than we thought or maybe we have less. Part of this uncertainty in how much we have to spend is due to not knowing exactly to what degree the carbon cycle will continue to bail us out. If needing to know exactly how much carbon we can emit is important then we need to know as much about the carbon cycle as we can. Otherwise, we may be well off the mark.
From towers in the forest to satellites
Some readers are likely familiar with what may be the iconic picture of the carbon cycle: the Keeling curve. This curve (pictured below) shows a steady upward march of carbon dioxide in the atmosphere. The sawtooth pattern captures the “breathing” of the Northern Hemisphere with spring pulling down CO2 from the atmosphere as plants regrow after winter and release it back to the atmosphere as they die or lose leaves in the fall.
As far as climate is concerned this is the important measurement as climate is forced to change in one direction or another in large part due to the concentration of excess carbon dioxide in the atmosphere. But this one curve misses many of the other important questions such as: Where is the other half (or so) of our emissions going if not into the atmosphere? Is the land uniformly taking up carbon or are there strong sinks in areas such as the northern mid-latitudes or the tropics? And how are these changing over time? This is just to name a few questions among the many.
The Mauna Loa observatory in Hawaii was the site of Charles David Keeling’s first measurements of atmospheric carbon dioxide in 1958. The inset graph shows the increase in carbon dioxide concentration and directly shows the carbon cycle in the “see-saws.” Photo credit: NOAA. Inset graph credit: NOAA and Scripps Institute of Oceanography.
Much of the information needed to answer these questions comes from the worldwide network overseen by NOAA, which includes flask air sample sites, tall towers, and aircraft. This network provides critical carbon cycle information at local and regional scales. Knowing which ecosystems and landscape types are sinks or sources for carbon and to what degree; and how these sources and sinks respond to disturbances, such as drought and wildfire, are questions that will help determine how the carbon cycle will evolve under continued carbon emissions and warming.
Whatever you do, don’t look down! Tall towers, such as this one operated by NOAA outside of Boulder, CO, are key components in the global network of carbon cycle measurements. For example, towers rising above dense forest canopies provide information about how the many gas exchanges and processes underneath influence what actually makes it in and out of the forest to the atmosphere. Photo credit: NOAA.
As of the last IPCC report, models generally show that the land and ocean will continue to take up emitted carbon, but climate change will likely reduce this service. There is uncertainty about how much this will decrease over the 21st century, which obscures how much we actually have in our “emissions” bank account. Reducing this uncertainty makes on-the-ground and in-the-air measurements that much more important.
Satellites, such as the Japanese GOSAT and NASA’s eventual OCO-2 provide carbon cycle measurements covering very large areas of the planet and are an important component to our understanding, but they still need to be validated with “in-situ” measurements, such as those currently under threat.
Carbon measurements on the chopping block?
Many important programs are on the chopping block across the sciences and, more broadly, in sectors across the country in these times of tight budgetary constraints. It would not be fair to comment on which should stay and which should go. That is Congress’ job.
I will say, though, that the NOAA greenhouse gas monitoring program’s budget is currently $6 million in a total federal budget that is in the trillions of dollars. This seems, relatively speaking, like a very small price to pay for information that is so central to our understanding of how the planet works and carries such large implications for society.
PS: For those wanting to learn more about the carbon cycle I highly recommend the general-audience accessible and informative book by David Archer, The Long Thaw.