Good news: the Earth is becoming greener! This fact still strikes me and requires digesting every time. Globally, plants are growing more than they are dying. This is called the ‘carbon sink’ over land, because the growing plants are taking up increasing amounts of carbon dioxide from the atmosphere and storing it through photosynthesis. This leads to more biomass, i.e., plant material, on Earth. The truly intriguing thing is that we are still lacking a scientific consensus about the location and nature of this global carbon sink, and therefore cannot predict its behavior in the future: will the plants keep growing at this increasing rate, or not?
How do we even know that the mysterious sink exists? Forests are certainly not popping up in our backyards! The answer lies in carbon cycle math. Anthropogenic emissions of carbon dioxide are well-known carbon sources to the atmosphere: burning of fossil fuel and cement production led to 9.9±0.5 PgC (petagrams of carbon = 1015 g) emissions in 2013, and the emissions due to deforestation and other land use change were 0.9±0.5 PgC. Luckily for us, not all of this carbon dioxide stays in the atmosphere. In 2013, oceans absorbed 27% of the emissions while 50% stayed in the atmosphere. Simple math leaves us with a residual sink of 23% – this is the work of the land sink, and means that on a global basis plants used this extra 2.5±0.9 PgC in photosynthesis for growing! However, the sources and sinks vary from year to year. Fig. 1 demonstrates how the land sink is paired with atmospheric increase of CO2: in some years, the plants absorb close to nothing, while in other years, plant uptake surpasses what is left in the atmosphere.
Figure 1. Temporal evolution of CO2 emissions and sinks. The land sink (green) is a residual of the sum of all sources minus the sum of the atmosphere and ocean sinks.
But where are we seeing this explosion in vegetation? This has been the hot potato, a true gold rush (or, more accurately, grant rush) of carbon cycle research for over a decade. The problem is not that the existence of the sink has eluded researchers but that the sink has been ‘discovered’ and relocated multiple times. For example, the sink has been located in North America, the Amazon, and in Europe. For a long time, the discussion was bouncing between two alternative major carbon sinks – boreal forests at high latitudes or rainforests in the tropics – until surprisingly, semi-arid regions in the Southern hemisphere were recently suggested as another potential carbon sink.
Perhaps a more important question than pinpointing the exact location of the sink is understanding the mechanism: why is there a net increase in plant biomass? At the moment, the plants are doing something we can’t: they suck in the carbon dioxide that would otherwise stay in the atmosphere and speed up the current rates of global warming with a contribution that might turn out unbearable. Wherever the sink is, we want to keep it! Moreover, we want to know if the sink is going to change in a changing climate, and how. For example, a sink in the tropics would likely be fueled by CO2 fertilization from the atmosphere, while a sink in the northern latitudes would likely be caused by forest regrowth or a northern expansion of the boreal forests. The significant difference is that some sinks are likely to become saturated while others won’t.
The challenge in locating the sink originates from the properties of the carbon dioxide gas itself. As this mesmerizing simulation shows, it is so well mixed and long-lived that local differences in its concentration are small, and do not necessarily tell anything about a local source or a sink because the gas may have been transported elsewhere. The search for sources and sinks is therefore a beautiful duet of carbon cycle models and measurements, where the model uses the measurements, prior assumptions, and atmospheric winds to backtrack the sources and sinks. The network of measurements has previously been too sparse for this pairing to work well enough but it is now expected to be revolutionized by the global CO2 measurements made by NASA satellite OCO-2, launched in July 2014. The mystery is about to be unveiled – it is only a matter of time.
I have found the Global Carbon Budget extremely useful for up-to-date information on the different aspects of carbon cycle and a very good collection of data sources (figures, PowerPoints, videos, etc.): http://www.globalcarbonproject.org/
More about the OCO-2 satellite mission: https://oco.jpl.nasa.gov/