Decline of ocean biomass
The phytoplankton mass has declined by 1% per year for forty years. The oceanic dead zones, deficient in dissolved oxygen produced by these unicellular organisms, spread at a rate of 8% per year: the United Nations Environment Program had 150 in 2003; they are more than 500 in 2015. Corals are also in danger and, with them, one billion human beings are threatened with starvation: according to a study conducted by Pascale Chabanet, researcher at the Institute for Research for Development (IRD ) of Reunion, on about sixty coral sites of the Indian Ocean, half has already disappeared; “With the extinction of the coral forests, it is the richest food bank of biodiversity on the planet that is dying out”.
Abrupt or irreversible consequences, and prospective
According to the Giec, “the global warming of the planet could lead to some effects that are abrupt or irreversible, depending on the pace and extent of climate change.”
- Sea levels are expected to rise by a few tens of centimeters by 2100, but over the next centuries and millennia the partial melting of polar ice caps could raise sea levels by several meters, flooding low-lying coastal areas, some low islands and deltas.
- About 20 to 30% of the species assessed to date are likely to be at increased risk of extinction if the increase in average global warming exceeds 1.5 to 2.5 °C (compared to 1980-1999) . With an increase in average global temperature of about 3.5 °C, model projections indicate extinctions (40-70% of species evaluated) worldwide. In May 2008, the United States listed the Alaska White Bear as an endangered species.
- The warming could induce an irreversible rebound effect at a human scale of time if it initiates forest fires and a large degassing of methane permafrost and seabed. The amount of methane currently released by melting permafrost is in the range of 14 to 35 million tonnes per year. It is estimated that this quantity will rise from 100 to 200 million tons per year by 2100, leading alone to a temperature rise of about 0.3 ° C. Over the next centuries, 50 billion tons of methane could be released by Siberian thermokarst lakes.
- Warmer and more acidic water, and more intense winter rainfall, as well as increased thermal shock and groundwater movement, may have indirect effects on the soil and subsoil by the end of the century: subsurface cavern collapses ( quarries, former underground shelters, slums of war or marl, etc.) are expected.
- Some, including climate scientist James Hansen, believe that “the Earth may have already exceeded the dangerous threshold of CO2, and the planet’s sensitivity to carbon dioxide is much greater than that retained in the models.”
Prospective optimistic and less optimistic visions coexist in 2009: some insist on the fact that technical solutions exist, and that it remains only to apply them (the houses could be isolated, and produce more electricity than they ‘consume, controlled transport, cities could be more autonomous and clean up the air).
Others – while inviting to apply these solutions as soon as possible, even a sustainable and friendly decrease – realistically, note that from 1990 to 2009, the trend was the achievement of high greenhouse gas emission ranges, leading to the IPCC disaster scenarios, and believe that it is time to stop talking about “change” to describe a disaster.
A study published in the journal Science of July 3, 2015 predicts, beyond 2 °C of warming, massive and generally irreversible impacts on ocean ecosystems and the services they provide; adaptation efforts would then become inoperative.
Very long-term phenomena
The majority of climatologists believe that the phenomena induced by the emission of greenhouse gases will continue and grow in the very long term. The third IPCC report emphasizes in particular the following points:
- some greenhouse gases have a long life expectancy, and therefore affect the greenhouse effect long after their emission (CO2 atmospheric lifetime of approximately 100 years);
- due to the inertia of the climate system, global warming will continue after the stabilization of greenhouse gas concentrations. This warming, however, should be slower;
- the still greater inertia of the ocean mass means that sea-level rise will continue even after the stabilization of the average temperature of the globe. The melting of ice caps, like that of Greenland, are phenomena occurring over hundreds or even thousands of years.
The recent observations in the Arctic zone conducted under the umbrella of the European Damocles program (Developping Arctic Modeling and Observing Capabillities for Long-term Environmental Studies) have created a real surprise in the scientific world. Indeed, these show a significant difference with the forecasts resulting from the various models and on which the IPCC conclusions are based: this results in a clear acceleration of the effects due to the increase of greenhouse gases in the Arctic ( total melting of pack ice in summer by 2020).
Scientists call feedbacks actions in return for the climate system on itself. These feedbacks are positive when global warming induces phenomena that themselves contribute to increasing this warming, and negative when the induced phenomena contribute to reduce the warming. Such feedbacks have already been observed during previous climate warmings, at the end of an ice age; the climate can thus, in a few years, warm up by several degrees.
The main feedbacks, which are positive, are:
- the release of methane: methane (CH4, which is nothing other than natural gas, with some “impurities”), is a greenhouse gas 23 times more warming than CO2. It is formed when the decomposition of organic matter takes place with a lack of oxygen, and under the action of bacteria, a process called anaerobic digestion. Wet soils (marshes) are very conducive to this creation of methane, which is then released into the atmosphere (this can give rise to spontaneous inflammation and we can observe wisps). If the soil is frozen, the methane remains trapped in the ice in the form of methane hydrates. The soil of Siberia is thus a huge reservoir of methane (probably too diffuse to be exploited industrially): according to Larry Smith of the Geography Department of UCLA, the amount of methane present in the Siberian soil would be 70 billion tons, a quarter of the methane stored on the surface of the planet. If the soil warms, the ice melts and releases the methane already present initially, which results in a stronger greenhouse effect, and consequently a runaway global warming, which melts the ice even faster. There is also talk of a carbon bomb;
- the slowdown and the modification of ocean currents: the ocean now captures one-third of the CO2 emitted by human activities. But if ocean currents slow down, the surface water layers can become saturated with CO2 and could no longer capture them as today. The amount of CO2 that a liter of water can absorb decreases as the water warms up. Thus, large amounts of CO2 can be released if ocean currents are changed. In addition, the accumulation of CO2 in the oceans leads to the acidification of the latter, which affects the marine ecosystem and can induce long-term release of CO2. The engines of the ocean circulation are of two types: the water getting closer to the poles cools and becomes denser. In addition, freezing seawater releases salt into liquid water (ice is made up of fresh water), becoming nearer to the ice caps even more dense. This water dives and feeds the pump: the warmer water from the surface is sucked up. Bottom water (cold) rises in the tropics and / or equatorial zones and warms up, in a cycle of more than 1000 years. If the ice caps melt, the pump crashes: indeed, the water that dives comes from the cap and no longer cooled water from the tropics. A similar effect is observed if precipitation increases at high latitudes (which is predicted by the models): the water that will plunge will be the fresh rainwater. In the long term, a strong disturbance of the Gulf Stream is possible;
- the variation of albedo: currently, the snow and the ice of the polar zones reflect the solar rays. In case of melting of this snow or ice, the sun’s rays are absorbed more, causing a further warming of these regions and a melting accentuated, amplifying the phenomenon.
Negative feedbacks are more uncertain:
- vegetation development: in some regions, global warming may be favorable for the development of vegetation, which is a carbon sink, which would help limit the increase in greenhouse gases; however, an article by a team of Dutch forest ecologists, published on Monday, December 15, 2014 in Nature Geo-science, announces that they have not observed any accelerated growth of tropical trees for a hundred and fifty years, which suggests that this negative feedback would not exist;
- the role of water vapor: global warming could increase the formation of clouds contributing to reflect more sunlight. However, water vapor is itself a greenhouse gas and the final balance of an increase in water vapor in the atmosphere is quite difficult to predict.
A study published in February 2018 evaluates the complex effects of climate change on clouds, which cover an average of 70% of the planet: it observes that clouds rise and cloud systems move generally towards the poles; both of these trends are expected to accelerate global warming; short-time observations suggest that tropical clouds will block less sunlight, thereby increasing global warming, and that thawing clouds may be a lower drag on warming than imagined; the amplifying effects of the greenhouse effect far outweigh the limiting effects.