Climate system: Land and forest
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Most rain falls near the Equator and at mid-latitudes, allowing rainforests and diverse ecosystems to thrive there. Land and vegetation also affect the climate: they influence the large-scale circulation by disrupting airflow, they absorb greenhouse gases, and they release moisture into the atmosphere through evaporation and transpiration.
The geographic patterns of climate dictate what type of vegetation can grow in a given region. Near the Equator lush rainforests are found, populated with trees and other plants that depend on the high year-round rainfall and temperatures. In cooler, drier or more changeable climates, vegetation is adapted to the local conditions. For example, the subtropics are home to trees able to resist regular droughts and a persistent arid climate, and high-latitude forests contain tree species adapted to cope with cold winter weather. Trees and other plants play a major role in the carbon cycle by absorbing carbon dioxide, acting as one of the largest carbon sinks in nature. Vegetation also affects the land’s albedo, tending to be darker than other surfaces and so absorbing more sunlight, which can influence local weather patterns.
Tropical rainforests cover just 2% of the Earth’s surface, but contain more than half of all known living species. The largest rainforest is the Amazon, stretching across nearly half of South America. This vast expanse of dense vegetation plays an important role in the Earth system, regulating both the carbon cycle and the water cycle. Trees act as a major carbon sink, absorbing carbon dioxide from the atmosphere through photosynthesis. Leaves also release water vapour into the atmosphere in a process called transpiration, affecting local and global weather by providing moisture for rain. The Amazon is the source of about 50–80% of its own rainfall. Changes in forest cover, for example deforestation caused by human activities, can have an impact on rainfall, wind and other weather patterns.
The general circulation of the atmosphere broadly consists of three large convection cells encircling each hemisphere. The Equator is characterised by moist, rising air and low pressure, the subtropics by dry, descending air and high pressure. But these patterns are far from uniform, because the world’s landmasses affect circulation in both the atmosphere and oceans. Continental areas warm and cool at a faster rate than oceans and the position of the continents determines the paths of ocean currents. The land also has a wider variety of surfaces than the oceans, leading to pronounced differences in albedo – from dark forests to white snow – and consequently the amount of sunlight absorbed. These factors produce many localised climate conditions.
Because of the oceans’ high thermal inertia, air masses over land warm and cool at a faster rate than those over the oceans. This leads to differences in seasonal temperature changes between continental and oceanic areas, which in turn play an important part in many weather patterns. In summer the continents warm up, leading to rising air and low-pressure systems. The low pressure draws in air from the areas of higher pressure over the cooler oceans, creating onshore ‘monsoon’ winds carrying moisture evaporated from the ocean surface. These monsoons provide a source of abundant moisture for heavy seasonal rainfall, particularly in Asia and parts of Africa. Without these rains, parts of southeast Asia would be desert.
The Himalayan mountain range was formed about 50 million years ago when the Indian landmass collided with the Asian landmass, causing the boundary between the two to buckle and forcing the ground upwards. The Himalayas are still rising, by about 1 cm each year, and have a pronounced effect on the climate and especially on the Indian summer monsoon. This monsoon is particularly intense because of the position of the mountain range, which creates a barrier to the monsoon winds blowing from the Indian Ocean. The moisture carried by the winds is forced upwards, causing it to cool and condense into clouds which often produce several metres of rainfall during the June to September wet season.
Large-scale average climate conditions are determined by the general circulation of the atmosphere and oceans. But on smaller scales, local climates are influenced by many factors, such as terrain and vegetation, which can create ‘microclimates’ that differ significantly from their surroundings. One example is mid-latitude deserts. Most deserts are in the subtropics, but desert conditions also exist at certain mid-latitude locations – such as Death Valley, the driest place in the USA. Situated at about 36 degrees north in the Mojave Desert, the valley is cut off from the Pacific Ocean by three mountain ranges. Moisture flowing in from the Pacific is forced upwards, cooling and condensing into clouds. As a result, almost all rainfall occurs on one side of the mountains, leaving Death Valley on the other side very dry.
Toby Marthews is trying to find out the impact that deforestation has on the carbon cycle. In one of the world’s largest ecological experiments, he meticulously measures the amount of carbon stored within the leaves, branches and soil in an area of Malaysian rainforest and monitors how it changes as it’s cut down and turned into a plantation. Combining the data he takes in Malaysia with data from around the world helps scientists understand the important role rainforests play in removing carbon from the atmosphere.
‘Working in rainforests is incredibly rewarding,’ says Toby. ‘Every day in the field is spent surrounded by a bewildering variety of animals and plants.’