The alarmed researchers have stated that forests and other land habitats currently consume 30% of the CO2 emissions of mankind, but in a few decades, accelerated global warming could turn these natural 'sinks' into carbon ‘sources' opening another formidable front in the battle against climate change. CO2 is also characterised as 'plant food' by climate sceptics, claiming that increased production of greenhouse gases would be offset by a huge upsurge in plant growth. The new research, however, reveals that the ability of plants to consume CO2 decreases above a certain temperature level, which varies according to area and species. In Science Advances this week, researchers announced that under current greenhouse gas emission patterns, plants around half the terrestrial environment of the globe will begin to emit carbon into the atmosphere at a much faster rate as compared to the when they sequester it by the end of the century. A team led by Katharyn Duffy from the University of Northern Arizona discovered that habitats that hold the most CO2, particularly tropical and boreal forests, could lose more than 45% of their capacity as carbon sponges by mid-century. Expected higher temperatures associated with elevated CO2 could degrade the absorption of land carbon, the study said, based not on modelling but on data gathered over a 25-year period. The researchers warned that failing to take this into account would contribute to a massive overestimation of the role of the Earth's vegetation that could play in reducing global warming. The terrestrial biosphere's temperature tipping point lies not at the end of the century or beyond, but over the next 20 to 30 years. The distinction between photosynthesis and respiration, two chemical processes central to plant life that react differently to rising temperatures, is crucial to understanding how this could happen. Plants consume carbon dioxide from their leaves and water from the soil by drawing energy from sunshine, creating sugar to fuel growth and oxygen, which is released into the air. This is photosynthesis, which can occur only while daylight is present. On the other hand, the conversion of energy by respiration to cells, with CO2 excreted as a waste product takes place around the clock. Duffy and her colleagues studied records from a global observation network, named FLUXNET, covering 1991 to 2015, in order to figure out whether there is a temperature above which land-based habitats will continue to consume less CO2. In essence, FLUXNET measures the CO2 flow between ecosystems and the environment. They find that, based on the type of plant, global photosynthesis increases at some temperatures, and then decreases afterwards. Respiration rates, however, arise without attempting to exceed a maximum threshold in all forms of environments. In comparison to sharply decreasing photosynthesis rates, respiration rates appear to increase at higher temperatures, the study found. This divergence could see the absorption of CO2 falling by half as early as 2040 if carbon emission remains unabated. The researchers concluded that we are quickly approaching temperature regimes where the productivity of the biosphere will decrease precipitously, challenging the potential feasibility of the land sink. The study states that the pillar of the 2015 Paris climate pact, capping global warming below two degrees Celsius above pre-industrial levels, allows for near-current biosphere productivity levels, retaining much of the land carbon absorption. So far, Earth has warmed at least 1.1 degree Celsius and is currently on track to heat up another two to three degrees by the end of the century unless emissions are cut quickly and significantly.
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