Huge implications for climate policy as new study confirms truth about land sink

It validates view that the prominent focus on the land sink is merely a smokescreen to mask countries’ hesitation at cutting down fossil fuel use

By Avantika Goswami
Published: Thursday 14 January 2021
The new study says that in boreal regions like the taiga, soil moisture is now more limiting than air temperature because of recent warming. This shows that the land sink is now less reliable as a climate change mitigator. Photo: Wikimedia Commons

Multiple scientific studies already confirm that the land sink cannot be relied upon to absorb as much carbon as needed to reach net zero emissions. A recent study by Songhan Wang, a doctoral student at the University of Nanjing, China and others, finds further evidence to support this. And the implications for climate policy are huge.

The land carbon sink (or terrestrial sink) has been under scrutiny in climate policy discussions recently owing to its inclusion in the roadmap to achieving net zero emissions by the middle of the current century. To reach net zero, emissions that are released must be balanced by an equivalent quantity of emissions absorbed.

And two ways to absorb carbon emissions have been identified by the climate community — through negative emissions technologies such as bioenergy with carbon capture and storage (BECCS) and through natural sinks such as forests, grasslands, etc. The latter are components of the land carbon sink, the magnitude of which scientists have been attempting to gauge for a number of years.

But there is considerable uncertainty, since the net exchange of carbon between land and the atmosphere is not in equilibrium. Human-induced factors such as deforestation and natural factors such as climate variability in sunshine, temperature and rainfall can cause a variation in the strength of the land carbon sink.

Rising global atmospheric concentration of carbon dioxide (CO2) due to the burning of fossil fuels increases the rate of photosynthesis in plants and can enhance carbon uptake from land-based sinks like forests and croplands. This is known as CO2 fertilisation.

According to the Global Carbon Budget 2019, this has caused an increase in the planet’s land sink in recent decades. Scientists say that global greening since the early 1980s may have reduced global warming by as much as 0.2-0.25 degrees Celsius (0.36-0.45 degrees Fahrenheit). In other words, the world would be even warmer than it is if not for the surge in plant growth.

However, increased CO2 also leads to other climate impacts — reduced precipitation, increased temperature, etc — that can weaken the land sink. In fact, the new study by Wang et al, finds that CO2 may be abundant, but moisture and nutrients are not.

According to Ben Poulter, study co-author and scientist at NASA’s Goddard Space Flight Center:

According to our data, what appears to be happening is that there’s both a moisture limitation as well as a nutrient limitation coming into play. In the tropics, there’s often just not enough nitrogen or phosphorus, to sustain photosynthesis and in the high-latitude temperate and boreal regions, soil moisture is now more limiting than air temperature because of recent warming. Thus, the terrestrial sink is becoming less reliable as a climate change mitigator

The researchers analysed datasets based on models, derived from satellites and consisting of multiple fields. They found that as levels of CO2 in the atmosphere increase, 86 per cent of land ecosystems around the world are becoming progressively less efficient at absorbing it.

This decreasing uptake of CO2 by the land sink could see a spike in the amount of CO2 remaining in the atmosphere after fossil fuel burning and deforestation and lead to a further reduction of the available carbon budget.

In its plan to become carbon neutral by 2060, China has included massive tree-planting efforts and the restoration of wetlands to absorb carbon. The European Union has come under fire for including carbon removals from natural sinks in its updated climate plan.

India’s Nationally Determined Contribution submitted towards the Paris Agreement also includes a pledge to create an additional, cumulative carbon sink of 2.5-3 gigatonnes of carbon dioxide equivalent through additional forest and tree cover by 2030.

It is increasingly clear that this prominent focus on the land sink, branded effectively as “nature-based solutions”, is merely a smokescreen to mask countries’ hesitation at decarbonising their economies and cutting down fossil fuel use to the extent required.

While the findings of this study may seem highly technical and specific, its implications are significant for policymakers; especially those who have been touting offsets through land sinks as a stop-gap to avoid meaningful cuts to gross greenhouse gas emissions.

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