Trees, forests, rainfall and climate change

The story: A new research found that converting agricultural land to forest would boost summer rains by 7.6% on average, in Europe. The researchers fo

Trees, forests, rainfall and climate change

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• The story: A new research found that converting agricultural land to forest would boost summer rains by 7.6% on average, in Europe. The researchers found that adding trees changed rainfall patterns far downwind of the new forests. Extra rain could partially offset the rise in dry conditions expected with climate change.
• A complex situation: The findings about increasing rainfall are partly based on observations of existing patterns. But the underlying reasons are less clear - they are probably related to the way the forests interact with cloudy air. Planting trees has become a major plank of many countries' efforts to tackle climate change all over the world. UK PM Boris Johnson said his country was aiming to plant some 30 million new trees every year by 2025.
• Turn agri into forests: The new research considers the impact of converting agricultural land across Europe to sustainable forests. It used an observation-based statistical model to estimate how changes to forest cover would impact rainfall across the continent.

1. The study found that if there was a 20% increase in forest, uniformly across Europe, then this would boost local rainfall, especially in winter and with greater impacts felt in coastal regions. But as well as local rain, the planting of new forests causes impacts downwind. The scientists found that rainfall in these locations was increased particularly in the summer months.
2. Taking the two impacts together, in what the team describe as a realistic reforestation scenario, they found that precipitation overall went up by 7.6% in the summer. That's quite a significant finding, and has implications for climate change.

• Climate change's biggest threat: The most threatening climate change signal expected in relation to percipitation, is a decrease in summer precipitation that is expected in the southern parts of Europe like the Mediterranean. Forestation would lead to an increase in precipitation, and hence will be very beneficial. The increased rainfall could have potentially negative impacts by boosting rainfall patterns that have already been affected by climate change, particularly in the Atlantic region.
• Clouds and forests: The reasons behind these local and distant impacts on rainfall are uncertain - scientists point out that the cloudy air that produces rain tends to stay longer over forested areas. And the rougher nature of these forests may trigger the rain, as a forest is a much rougher surface than agricultural land. So it induces more turbulence at the land-atmosphere interface, and the forest exerts more drag on to the atmosphere than agricultural land. The new forests tend to evaporate more moisture to the atmosphere than agricultural land and this extra supply is the main reason behind increased rainfall downwind. The fact that trees planted in one country may have implications in another means that the world should really consider all the impacts of how it uses land.
• Not so direct: It also shows once again, that the idea of solving climate change with trees is not as simple as it is often portrayed. Planting trees is certainly not a quick fix for climate change. Adding new trees or restoring lost forests can never compensate for the greenhouse gas emissions arising from the burning of fossil fuels. That remains the primary issue.
• Summary: Even just cutting back on emissions won't be enough, and the world will need to actively remove carbon from the atmosphere if warming is to stay below 1.5 C. Tree planting is a potential candidate for generating these negative emissions, but planting trees should never be an excuse for not acting on reducing carbon emissions by all means possible.

 

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