Forests could soon be drivers of climate change

The carbon cycle consists of a set of natural mechanisms that together process huge quantities of carbon dioxide. These mechanisms involve both the capture of carbon dioxide and its conversion to organic molecules in photosynthesis, and its release into the atmosphere through natural processes (respiration). Carbon sinks include both the oceans, where dissolved carbon dioxide is captured and transported by ocean currents such as the Atlantic Meriodional Overturning Circulation (AMOC), and converted into biomass by phototrophic bacteria; and land sinks, including both forests and soil.

Changes in the atmospheric concentrations of greenhouse gases have the potential to unbalance these vital natural cycles. Increasing CO₂ concentrations tend to cause increases in the rate of growth of plants, which would tend to mean that the capacity of soils for the storage of carbon would increase; however, experimental studies have found that changes to atmospheric CO₂ concentrations can also cause accelerated microbial activity in soil, for example, with the consequence that a potential carbon sink can become a carbon source.

The balance between photosynthesis (capture of atmospheric carbon dioxide) and respiration (release of CO2 back into the atmosphere) is quite a delicate one. As long as the rate of respiration is smaller than the rate of photosynthesis, the amount of carbon stored in the land sink will increase; once the rate of respiration exceeds the rate of photosynthesis, the carbon stored in the land sink will gradually be released into the atmosphere.

Duffy et al recently reported the findings of a large-scale experimental project in which a large consortium of scientistsmeasured the carbon flux through a massive number of sites around the world. Their aims were to investigate the dependence of the rates of photosynthesis and respiration on temperature, to determine the temperature at which the capacity of the land sink to absorb carbon would decline, and also to estimate the timescale on which such a tipping point might be reached.

Fig. 1 Temperature dependence of global carbon fluxes.

Because data were collected at sites around the world, it was possible to model behaviour across a very large temperature range. The findings were striking. For C3 plants, which make up the majority (~90%), the rate of photosynthesis increased up to 18°C, but then declined thereafter; for C4 plants, which are found predominantly in very hot conditions, the rate of photosynthesis peaked at 28°C and again declined thereafter. In contrast, the rate of respiration increased continuously across the range of temperatures studied (up to 38°C), with no evidence of a peak.

Fig. 2 Temperature dependence of the terrestrial carbon sink.

The photosynthesis data were combined to yield a plot showing the total land sink photosynthesis and respiration data. It makes for worrying reading. The grey region in the plot shows the mean annual temperature range from 1991 to 2015. However, the temperature is not uniform all year round in most of the world; during the warmest months the mean temperature exceeds the point at which photosynthesis begins to decline, and at the same time, the rate of respiration increases dramatically. Moreover, the distribution of carbon sinks around the world is non-uniform; sinks that store a large fraction of the Earth’s carbon (e.g. the Amazon) are in regions for which mean temperatures are already close to the point at which the rate of photosynthesis begins to decline. Thus, Duffy et al conclude that

“the tipping point of the terrestrial biosphere lies not at the end of the century or beyond, but within the next 20 to 30 years… we will cross the temperature threshold of the most productive biomes by midcentury, after which the land sink will degrade to only ~50% of current capacity if adaptation does not occur…. Failure to implement agreements that meet or exceed limits in the Paris Accord could quantitatively alter the large and persistent terrestrial carbon sink, on which we currently depend to mitigate anthropogenic emissions of CO₂ and therefore global environmental change.”

The message is clear. We must act fast; failure to do so may condemn us to catastrophic climate breakdown. However, the last couple of years have seen the beginning of a revolution that offers the promise of a solution to our problem: large scale generation of solar, wind and hydro-electric power. The electrification revolution promises a means to decarbonise energy, save us from climate breakdown, improve the quality of our lives and end once and for all the chaos and inequality of a world ruled by fossil fuel elites.