We already know that over the next hundred years the earth will be warmer. The average global temperature in 2020 was about 14.9 degrees C, 1.2 (0.1) degrees C above the preindustrial (1850-1900) level. The cost of climate change would be quite different if the planet got warmer by 1.5 degrees C, or 2 degrees C, or more. According to the October 2018 report of the Intergovernmental Panel on Climate Change (IPCC), 70 per cent of coral reefs would vanish if the planet got warmer by 1.5 degrees C and 99 per cent at 2 degrees C.

The number of people directly impacted by the rise in sea levels and the transformation of cultivable land into desert would also be quite different under two scenarios. The Emission Gap Report 2020, released recently by the United Nations Environment Programme states unequivocally that human induced greenhouse gas emissions continue unabated.

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Climate change is caused by the accumulation of man-made CO2 and other greenhouse gases in the atmosphere. CO2, the most significant of all greenhouse gases in the atmosphere has inexorably mounted with huge consumption of fossil fuels. In 2016, almost four-fifths of world energy was based on fossil fuels. The average estimate implies that when income of an individual increases by 10 per cent his/her CO2 emissions increases by 9 per cent.

There are number of feedback processes important to Earth’s climate system and, in particular, its response to external radiative forcing. The global carbon cycle comprises the most important set of climate feedbacks. In particular, the two main reservoirs of carbon in the climate system are the oceans and the terrestrial biosphere (the life supporting part of the surface). These reservoirs have historically taken up large amounts of anthropogenic CO2 emissions.

Over time-spans of decades to centuries, plants play a major role in pulling CO2 out of atmosphere. Such has been the case since about three billion years ago, when oxygenic, or oxygen-producing, photosynthesis evolved in cyanobacteria, the world’s most abundant type of single-celled microscopic marine algae called phytoplankton (also known as macroalgae). These organisms ~ including diatoms and other algae-inhabit three quarters of the earth’s surface. They are similar to terrestrial plants in that they contain chlorophyll and require sunlight in order to live and grow. Phytoplankton and all land dwelling plants ~ which evolved from phytoplankton about 500 million years ago ~ take up CO2 from atmosphere and water from the surroundings and use energy from sunlight to split water molecules into atoms of hydrogen and oxygen. The oxygen is liberated as a waste product and makes possible all animal life on earth, including our own. The global carbon cycle (to a large extent, its climate) depends on the photosynthetic organisms using hydrogen to help convert the inorganic carbon in CO2 into organic matters ~ the sugar, amino acids and other biological molecules that make up their cells.

Every drop of water in the top 100 meters of ocean contains thousands of free-floating phytoplankton. Its activity occurs mostly within the first 50 meters of the surface which varies widely according to the season and location. The role of phytoplankton in the carbon cycle is quite different from that of trees and other plants, which absorb CO2 and serve as a storehouse, or ‘sink’, of carbon. Instead, ocean life absorbs CO2 during photosynthesis and, while most of the gas escaped within about a year, some of it is transported down into the deep ocean via dead plants, body parts, faeces (of zooplankton) and other sinking materials. The CO2 is then released into the water as the materials decay, and most of it gets absorbed into ocean water by combining chemically with water molecules. Although a small but possibly significant percentage of the sinking organic materials becomes buried in the ocean sediments, most of the dissolved CO2 is eventually returned to the surface via ocean currents ~ but this can take centuries or millennia. When gas gets released below about 200 meters, CO2 stays for much longer because the colder temperature ~ and higher density ~ of this water prevents it from mixing with warmer water above. Through this process, known as the biological pump of carbon cycle (or also called the marine carbon pump), phytoplankton remove CO2 from surface waters and atmosphere and store it in the deep ocean. It is estimated that the material pumped into deep ocean amounts to between seven billion and eight billion metric tons, or 15 per cent of the carbon that phytoplankton assimilate every year.

The rate at which phytoplankton consume CO2 and convert it into sugar etc. for producing tissue and energy varies enormously. This makes it difficult to sample and estimate their annual consumption of CO2. In the second half of the 20th century, biological oceanographers made thousands of individual measurements of phytoplankton productivity. But data gathered were scattered and extremely small. Even estimates of total global productivity using mathematical models were unreliable. However, in 1977, NASA launched the Sea Wide Field Sensor (Sea WIFS), the first satellite capable of observing the entire planet’s phytoplankton population every single week. Although various research groups differed in their calculations, they had all arrived at the same startling conclusion: every year phytoplankton incorporate approximately 45 billion to 50 billion metric tons of inorganic carbon into cells.

There are some factors related to ‘feedback loops’. For example, sometimes Di-methyl sulfide (DMS) emitted by phytoplankton promotes the formation of clouds over oceans; changes in plankton population could lead to changes in cloudiness. At the same time, more clouds reduce the amount of solar radiation reaching the oceans which could reduce plankton activity. Another possible feedback could occur near the poles. When global warming causes sea ice to melt more light would reach and warm the surface waters, either benefitting or damaging certain plankton. In case burning of fossil fuels returns CO2 to the atmosphere about a million times faster, marine phytoplankton and terrestrial forests cannot naturally incorporate CO2 quickly enough to mitigate this increase. As a consequence, the global carbon cycle falls out of balance, warming the planet. Again, if all the world’s marine phytoplankton, popularly called hidden or invisible forest, were to die today, the concentration of CO2 in the atmosphere would rise by 200 parts per million ~ or 35 per cent ~ in a matter of centuries. To face these eventualities several researchers and private corporations proposed that phytoplankton photosynthesis and the biological pump could artificially be enhanced in two ways: by adding extra nutrients (dilute iron solution) to the upper oceans or by ensuring that nutrients not consumed are used more efficiently.

Importance of phytoplankton could hardly be overemphasized because it draws nearly as much CO2 out of the oceans and atmosphere as all land plants do. New satellite sensors are now watching phytoplankton populations daily. The idea of designing commercial ocean-fertilization projects is still under debate among the scientific community and policymakers. It seems ironic that society would call on hidden forests to help to solve a problem created in parts by the burning of their fossilized ancestors.