“The Earth is an unimaginably intricate system comprised of countless subsystems – many of which are poorly understood and some, probably, still unknown. The science that purports to explain those systems is unsettled and, well…, complicated.”
It’s complicated. Trees are natural carbon sinks, absorbing carbon dioxide from the atmosphere. But they also emit volatile chemicals that can form methane and ozone, both powerful greenhouse gases. A 2014 study suggests that deforestation between 1850 and 2000 may have reduced volatile chemical emissions enough to “slightly offset the warming from greenhouse gas emissions.”
But it’s more complicated than that. Trees’ chemical emissions can also form aerosols that impact the climate in different ways. For instance, they can create a haze that reflects sunlight back into space. Aerosol particles may also “act as seeds for cloud droplets,” which similarly reflect sunlight away from the Earth.
Complicating the picture still further, land covered with trees tends to be darker than land covered with grass, sand, or snow. Dark surfaces trap more of the sun’s heat, warming the local climate. Does the CO2 that trees absorb offset the heat they absorb? It depends. According to a 2007 study, planting trees in the tropics – where they grow quickly and trap a lot of CO2 – has a net cooling effect. However, planting them in temperate areas between latitudes 50o and 60o (roughly between the U.S.-Canadian border and Fairbanks, Alaska) has either no impact, or, according to a 2022 study, a net warming effect.
The United Kingdom, all of which lies between 50o and 60o latitude, is planning to spend some £750 million ($975 million) to reforest the country by 2025. Net Zero Watch, the Global Warming Policy Forum‘s website, reports that the 2022 study “suggests that the Government’s plans are not only wasteful – and often detrimental to the environment and food security – but may actually increase global temperature, the opposite of the intended effect of the Net Zero project.” 
If global warming truly is an existential crisis, our solutions should, at the very least, not make it worse. Moreover, costly solutions that have little, no, or negative impacts will quickly sour the public on further ventures. An expensive misstep may be the last step a government can take in the face of angry voters.
Suppose, however, that after due research and consideration, we determine that planting trees in a certain location is beneficial. What types of trees should we plant? According to one British tree specialist,
Broadleaved species – such as oak, beech and maple – are best because they have a larger surface area of leaves which generates more photosynthesis, whereas conifers absorb more heat.
Planting a diverse mix of species will maximise the benefits of trees, as well as helping with conservation efforts such as insect and wildlife initiatives, and ensuring the landscape is better prepared for pests or diseases which could wipe out an entire species of tree in one area.
But trees absorb large amounts of groundwater, which could place an unsustainable burden on a region’s water supply. Species that can survive on little water may be the best choice.
On the other hand, elevated levels of CO2 in the atmosphere improve plant water-use efficiency, which will reduce, if not eliminate, the impact on groundwater. In addition, once established, trees can affect the water cycle and increase area rainfall. On yet another hand, while “overall, the impact of trees on rainfall is expected to be positive… it can have complex ramifications.”
The mutual relationships between forests and climate are actually really rather more complex and not fully understood.
The Earth is an unimaginably intricate system comprised of countless subsystems – many of which are poorly understood and some, probably, still unknown. The science that purports to explain those systems is unsettled and, well…, complicated.
 The analysis herein is applicable to the Trillion Trees Act of 2021 and like proposals as a climate strategy.