The recent Paris Climate talks concluded with an agreement to “[hold] the increase in the global average temperature to well below 2 degC above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 degC above pre-industrial levels.” Expressing these targets in terms of temperatures is appealing as it is the change of temperature rather than change in atmospheric carbon dioxide concentration per se that has a direct impact on us. But how do we know what levels of emissions of carbon dioxide (and other greenhouse gases) will keep us within these targets and, perhaps more importantly, how accurately do we know this?
The carbon cycle is one of the key players in global climate change. Sequestration of carbon dioxide (CO2) from the atmosphere by the oceans and the land surface combined soaks up approximately 50% of anthropogenic emissions. Without this the rate of change of CO2 in the atmosphere would be nearly double and the rate of climate change significantly faster than it is today. What is particularly remarkable about this natural sequestration is that the proportion of carbon being sequestered has remained relatively constant over time despite increasing CO2 concentrations in the atmosphere. In other words the total amount being absorbed by the oceans and atmosphere has been increasing.
The processes that control the uptake of carbon by the land surface are very poorly understood. Indeed there remains some debate as to why the sequestration by the land surface has been able to keep pace with the increasing CO2 levels in the atmosphere. The most likely explanations for this are some combination of increased levels of nitrogen deposition from industrial processes (which allows plants to photosynthesise more), the simple availability of more CO2 for the plants to take up (so-called carbon fertilisation) and the regrowth of forests in regions where they had previously been cleared (primarily in northern latitudes). However we do not fully understand if these different elements will continue to function in the same way as the climate changes. Will the land surface continue absorb the same proportion of emissions or will it become a less strong sink? Some modelling studies even suggest the land surface could become a net source of carbon as the world warms, thus acting as an exacerbating feedback to climate change.
Just prior the Paris talks the Global Carbon Project released their annual budget describing the state of global carbon cycle. They conclude that to keep below 2 degC rise emissions will need to drop to near zero very soon. Figure 1, produced by the Global Carbon Project, shows the ensemble of emissions scenarios taken from the Intergovernmental Panel on Climate Change (IPCC) with current global emissions superimposed. We are currently tracking near to the mean of the most aggressive of the scenarios that are predicted to give a change in temperature of a range 3.2 to 5.4 degC by the end of the century. It is important to realise, however, that for any given scenario there is still a considerable uncertainty in the resulting temperature change – and a large part of that uncertainty comes from our lack of understanding of the land surface.
The Paris climate agreement explicitly acknowledges the potential of “sustainable management of forests and enhancement of forest carbon.” This is undoubtedly an important component of any strategy to limit climate change. However if we want to understand the future, and thus be able to make well informed choices about emissions and mitigation, it is critical that we continue research into the carbon cycle and in particular the role of the land surface.
My group in the Department of Meteorology at the University of Reading are involved in numerous projects that seek to improve our understanding of the global carbon cycle. In the MELODIES project our role is the production of new land cover data sets optimised to detect land cover change, a significant factor in anthropogenic emissions of CO2. We are part of the PalEON project, which is testing the way in which key land surface models represent carbon processes that play out on long time scales (such as change in plant species composition). With Forest Research we are working to understand the errors in observations of forest carbon balance, which is critical if we want to use these data to test models with. As part of the National Centre for Earth Observation we are working on using satellite observations of the land better to constrain predictions of the global carbon cycle. And last, but not least, we have several projects working on the development and testing of JULES, the land surface component of the Met Office Unified model – a key part of UK climate predictions that go into the IPCC reports.
Figure 1. Emissions scenarios for various Representative Concentration Pathways (RCPs) with historical and current emissions overlaid. Taken from: http://www.globalcarbonproject.org/carbonbudget/index.htm