Dr Willow Hallgren
The way that humans use and change the land and its vegetation can have a profound effect on the climate, and can either counteract or exacerbate climate change. These effects are often overlooked when discussing action on climate change (aka global warming or global heating).
The recent public outcry over land clearing rates in Australia, particularly Queensland, was largely about the resulting loss of biodiversity. Less talked about was the impact of all that land clearing and deforestation on local and regional climates, and the impact of this on human communities and surrounding ecosystems as global heating intensifies.
Global heating impacts natural ecosystems and agriculture
Human activities are heating the planet, and our climate is changing, causing major changes to both natural ecosystems and agriculture. Increasing average temperatures will shift the locations of what crops can be grown where, and will make land in some locations unable to support agriculture. More extreme temperatures are also drying out ecosystems and endangering wildlife (like flying foxes who can’t handle heatwaves and start to drop out of the trees, dead).
Longer and more severe droughts are damaging not only crops but entire ecosystems by contributing to increased bushfire severity.
Observed changes in the amount, and timing of rainfall, and more extreme storms and floods are putting crops and livestock at risk. Longer and more severe droughts are damaging, not only crops, but entire ecosystems by contributing to increased bushfire severity.
The 2019–2020 summer will long be remembered as illustrating the terrible impact of drought, made even more severe by climate change, which caused unprecedented mega-bushfires that burned throughout much of Australia.
Even in the normally green, well-watered Northern Rivers region we have endured the sight of stressed and dying wet-eucalypt forests in many places and the burning of areas of ancient Gondwana rainforests that are not supposed to burn.
Creating local climates
Most people know that climate broadly determines where ecosystems occur: think of rainforests along the equator, and conifer forests and tundra in the higher northern latitudes.
What is less well known, is that some ecosystems can create their own climate; if you wander into a densely canopied forest, like a rainforest, on a hot day you will find that the air underneath the canopy is relatively cool and moist compared to the air outside; this is because forests have their own microclimates. The forest canopy shades the ground from the heat of the sun, minimising ground evaporation and protecting soil moisture. Trees also contribute to lower temperatures by adding humus, having roots that can tap into deeper stores of water in soil, and they transpire this through their leaves into the air. This moisture cools the air, which is why large expanses of forests provide a regional cooling effect.
Sufficiently large areas of forest can also create their own self-sustaining climate. They do this by actively drawing in air and moisture from surrounding regions that have less vegetation through a process called ‘the biotic pump’; this is what occurs over the Amazon rainforest.
Fewer trees means less rainfall
Land clearing and deforestation can change the climate in many ways. Deforestation removes the canopies that shade the ground, which can increase evaporation and dry out the top soil layer. Fewer trees means less water is transpired from the deeper soil into the atmosphere, reducing evaporative cooling of the air. At regional scales, if we clear large areas of forest, we disrupt the rainfall-creating ‘biotic pump’ mechanism. It is well documented that deforestation often correlates with some decline in rainfall and cloud cover, as has been seen in the Amazon.
The loss of vegetation and forest cover will exacerbate climate change
Trees and other vegetation also hold the soil together and prevent significant runoff and soil erosion when it does rain; this assists in recharging the soil moisture. The take-home message is – the loss of vegetation, and particularly forest cover, will exacerbate the warming and drying trends of climate change that we are currently witnessing across much of Australia.
Reafforestation for climate management
One solution, which will both mitigate climate change, and help us to adapt to it, by counteracting the warming and drying trend, would be to preserve as much forest as possible, and to increase forest cover. This is particularly important close (but not too close, owing to bushfire risk) to cities and agricultural areas, so that the cooling effect of forests can benefit these areas. Some scientists refer to this proposal as a type of regional geoengineering.
There are studies that show that planting large numbers of trees is one of the cheapest, easiest ways to remove carbon dioxide from the atmosphere, which will slow down global heating. Given dense, shade-creating canopies can take decades to grow, we should start planting trees now, to (a) absorb as much CO2 as possible, to limit further global heating, and (b) take advantage of our current relatively stable climate, before it becomes too dry in areas where it is currently possible to grow these forests for climate regulation.
Reflecting heat to lessen extreme temperatures
Besides using trees and forests to counteract the higher temperatures, we can make both rural and urban land surfaces more reflective of sunlight (ie increase the land ‘albedo’) to lower daytime temperatures. Examples of ways to increase the amount of sunlight that agricultural areas reflect (and thereby reduce heat) include no-till farming, changing the crop phenology and timing of practices (eg double cropping), bioengineering more reflective crops, and using greenhouses.
In urban areas, the cooling effect of an urban forest can be enhanced by making hard surfaces like roofs, roads and pavements more reflective of sunlight. For example, painting roofs white, and having solar panels on roofs can decrease the daytime heating of urban areas as well as generating electricity without the use of fossil fuels – a win-win!
Combating the climate change emergency will take all the weapons in our arsenal, including the ways we use the land and its ecosystems. Ordinary citizens can start now, by seizing some of the many low-cost, low-hanging fruit of climate action: planting trees, painting your roof white, and using solar panels to generate renewable energy.
More reading
The Conversation: Our cities need more trees, but some commonly planted ones won’t survive climate change
NASA: Examining the Viability of Planting Trees to Help Mitigate Climate Change
Author
Dr Willow Hallgren is an earth-system scientist who studies the impact of climate change on ecosystems and biodiversity, the feedbacks between vegetation and the climate, and how policy can influence climate change, by changing how we use the land.
Willow has previously worked as a climate and biodiversity scientist in government, industry, and academic roles in both Australia and the USA at the Massachusetts Institute of Technology (MIT). She was also previously the Science editor of Monash University’s student newspaper Lot’s Wife.
She is a city escapee of many years now and is currently hiding out among the hill tribes of the beautiful Tweed Valley.
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