With one year to go until COP26 in Glasgow, and today’s launch of Together For Our Planet, Met Office Chief Scientist Professor Stephen Belcher reviews the influence tropical deforestation is having on the journey to a resilient Net Zero through the significant role they play in balancing atmospheric CO₂ concentrations, storing carbon and changing the water cycle.  

We all learn about the basic principles of photosynthesis at school, the importance of plants providing the oxygen we breathe and also how they absorb carbon dioxide. So, on a simple level we all understand how important the ‘lungs’ of the earth are to enable all forms of life. But plants are also playing an important role in global climate change and the challenge to keep warming to manageable levels.  

Tropical forests cover a remarkable area, something like 12% (17 million km²) of the earth’s land surface. With this scale they not only cycle vast amounts of the oxygen that supports life, but are also incredibly important sinks of carbon from our atmosphere. Around 200 gigatons of carbon are stored in tropical forest vegetation, with further carbon stored in soils. If the carbon in the standing timber were released it would be equivalent to emitting 667 gigatons of CO₂, which is equivalent to all fossil emissions of CO₂ since about 1997. And as atmospheric CO₂ concentrations increase as a consequence of human activity, tropical forests are likely to act as stronger sinks for carbon.  

So, deforestation reduces the amount of carbon that forests can store, meaning that a higher proportion of future CO₂ emissions from fossil fuels will remain in the atmosphere, leading to larger changes in climate. And deforestation is a double whammy: by cutting down these trees we also release CO₂ emissions previously stored in the forest system. 

Deforestation also affects the climate by altering surface distribution of radiation, water and heat, so-called non-CO₂ effects.

Tropical forest is often replaced by crops and pasture, which tend to be lighter in colour than forests, which means that after deforestation more sunlight is reflected from the land back into space. This reduced absorption of sunlight leads to cooling.  

However, when plants take in CO₂ from the air they transpire, they lose water to the air, which carries heat away from the surface. Crops and grasses tend to transpire less than forests, meaning that after deforestation heat is less efficiently lost from the land surface and the surface climate is warmer. 

The numbers are delicate. At high latitudes deforestation causes a large cooling effect from increased reflection of sunlight and a small warming from reduced transpiration, resulting in a net cooling at the surface. However, in the tropics, where deforestation rates are currently highest, the transpiration affect is much larger, and deforestation causes a net warming at the surface. 

Although not as important on a global scale, these non-CO₂ effects are one of the most important contributors to warming near the location of deforestation. This effect does have global implications because warmer local temperatures reduces the ability of the forest to absorb atmospheric CO₂ by photosynthesis, leading to a warmer background global climate.  

There is also moisture recycling and land management to consider. For example, different crops can have varying responses to factors such as drought, and ploughing releases further CO₂ emissions from the soil, reducing the carbon sink still further. Deforestation can also impact the rainfall in other parts of the forest, meaning they may not get the rainfall they need to stay healthy and resilient. It may even affect rainfall patterns elsewhere in the world.

It’s important to understand the scale. Statistics from the UN’s Food and Agriculture Organization (FAO) show that, at its peak, the rate of deforestation in the tropics reached 95,430 km² per year. This has gradually decreased from 1990 to 2015, dropping to 55,200 km² per year. But that is still around a football pitch every 4 seconds for the 2010-2015 period, or all of the forests in the UK gone in about 7 months! Concerningly deforestation of the Amazon has increased, particularly in 2019 which saw the highest rate in a decade.  

So what does this all mean in the context of net zero, the target that nations around the world are racing towards to balance up their carbon emissions and not contribute further to human induced climate change?  

Well, its first important to say that net zero will not be achieved through limiting tropical deforestation alone. Ultimately net zero can only be achieved by substantially reducing the use of fossil fuels. However, net zero will be made easier if tropical deforestation is limited.

While net zero is an important target, it is not only climate change that will benefit from stopping deforestation. Forests are important habitats, are culturally important and provide other ecosystem services. So there are a multitude of benefits from taking the same action.  

Positive steps are being taken. For example, Brazil’s stated Intended Nationally Determined Contribution (INDC) to the UN’s climate change treaty includes eliminating illegal logging and restoring 120,000 km² of forest by 2030. 

So what can science do to play a part? We need to quantify the effects of deforestation on climate, and work at the Met Office is doing just that. We have developed with UK academic partners the UK Earth System Model (UKESM1) to simulate two-way feedbacks between forests and climate, including the impacts of deforestation and the impact of climate change on forest resilience.  

Alongside this large-scale perspective, we are also working to better understand the small-scale processes that drive the processes, using the JULES land-surface model to simulate forests at key observational sites. Much of this work is being done under the CSSP Brazil program in partnership with Brazilian colleagues at INPE, INPA and CEMADEN, and with UK academic partners. 

Ultimately, we are using our earth system modelling to assess the impacts of future deforestation and mitigation on climate and to calculate carbon budgets that avoid the worst impacts of climate change. With this scientific information policy makers can make informed decisions on how they will act when it comes to deforestation.