Climate projections – Climate models for global warming

In the fifth IPCC report, a climate projection is defined as the climate response to a greenhouse gas and aerosol concentration emission scenario. These projections are simulations obtained from numerical models. They depend heavily on the scenario of gas emission and greenhouse effect, itself based on assumptions about future socio-economic and technological development. This emission scenario is the most important source of uncertainty in projections of the global atmospheric surface temperature by 2100. These projections are made in the framework of the intercomparison project of the models coupled by the research groups of the whole world. The last year of this project (CMIP5) involved nearly 50 climate models.

Climate models

The projections by the scientists of the future evolution of the climate is possible by the use of computerized mathematical models treated on supercomputers. These so-called general circulation models are based on the general laws of thermodynamics and simulate the movements and temperatures of atmospheric and oceanic masses. The most recent ones also take into account other phenomena, such as the carbon cycle.

These models are considered valid by the scientific community when they are able to simulate known climate variations, such as seasonal variations, the El Niño phenomenon, or the North Atlantic Oscillation. The most recent models satisfactorily simulate temperature changes during the 20th century. In particular, the simulations carried out on the climate of the twentieth century without integrating the human influence do not take account of the global warming, while those including this influence are in agreement with the observations.

Computer models simulating the climate are then used by scientists to establish scenarios of future climate evolution, but also to identify the causes of the current global warming, by comparing the observed climate changes with the changes induced in these models by different causes. natural or human.

These models are subject to uncertainties of a mathematical, computer, physical, and other nature. The three main sources of uncertainty mentioned by climatologists are:

• cloud modeling;
• the simulation of small-scale phenomena, such as thunderstorm cells, or the effect of relief on atmospheric circulation;
• the modeling of the interface between the oceans and the atmosphere.

In a more general way, these models are limited on the one hand by the computational capacities of the current computers, and the knowledge of their designers on the other hand, the climatology and the phenomena to be modeled being of a great complexity. The importance of the necessary budgetary investments is also a significant aspect of research in the field of global warming. Despite these limitations, the IPCC considers climate models as relevant tools for providing useful climate change scenarios.

Continuing global warming

For climate scientists grouped under the IPCC, temperature increases are expected to continue in the 21st century. The magnitude of the most likely warming is expected to be 1.8 to 3.4 °C at the end of the 21st century (which does not mean the end of the temperature increase in 2100).

The magnitude of the predicted warming is uncertain; the simulations take into account:

• model uncertainties;
• uncertainties about the behavior of humanity in the 21st century.

To take into account this last parameter in the projections, climatologists use different scenarios for greenhouse gas (GHG) emissions and aerosols. The scenarios used in the 3rd and 4th IPCC reports are detailed in the Special Report on Emissions Scenarios (SRES) report. These scenarios do not take into account the possibility of an intentional change in GHG emissions on a global scale. The scenarios used in the fifth phase of the CMIP project as the basis for the 5th IPCC report are called RCP scenarios for representative concentration pathway. Four scenarios were used:

• RCP 8.5: high emission scenario. This scenario corresponds to a future without a policy to reduce greenhouse gas emissions. It is comparable to the old SRES A1F1 scenario.
• RCP 6: intermediate emission scenario. In this scenario, radiative forcing stabilizes shortly after 2100, which corresponds to a strategy to reduce GHG emissions. It is comparable to the old SRES B2 scenario.
• RCP4.5: intermediate emission scenario. Like the RCP6 scenario, it corresponds to a strategy to reduce GHG emissions. It is comparable to the old scenario SRES B1.
• RCP2.6: Reduction of GHG emissions. It is an ambitious policy to reduce GHG emissions. It does not have its equivalent among the old SRES scenarios.

Uncertainties related to the operation of the models are measured by comparing the results of several models for the same scenario, and comparing the effects of small modifications of the emission scenarios in each model.

The variations observed in the climate simulations are at the origin of a scattering of the projections of the order of 1.3 to 2.4 ° C, for a scenario (demographic, growth, “global energy mix”, etc. .) given. The type of scenario envisaged has an effect of around 2.6 °C on the global warming simulated by these models and explains a large part of the margin of uncertainty existing as to the extent of future warming.

The IPCC 2100 horizon temperature increase projections (2007 SPM) range from 1.1 to 6.3 °C. IPCC experts are refining their projections with values ​​considered “best estimates”, which reduces the range from 1.8 to 4.0 °C. And by eliminating the A1F1 scenario, considered unrealistic, the increase in temperature would be between 1.8 and 3.4 °C.

The four scenario families of the fourth report and the forecasts of average global temperature increases in 2100
Source : GIEC	More economic goals	More environmental goals
Globalization (Homogeneous world)	A1 Rapid economic growth (groups: A1T/A1B/A1Fl) 1,4 — 6,4 °C	B1 Overall environmental sustainability 1,1 — 2,9 °C
Regionalization (Heterogeneous world)	A2 Economic development with a regional focus 2,0 — 5,4 °C	B2 Local environmental sustainability 1,4 — 3,8 °C

Giec scientists consider that these scenarios are the best projections currently possible, but that they are still subject to readjustment or reconsideration as and when scientific progress is made. They consider that it is necessary to obtain more realistic models and a better understanding of climatic phenomena, as well as associated uncertainties.

However, many climate scientists believe that short-term improvements in climate models will not fundamentally alter their results, namely that global warming will continue and its magnitude will be greater or smaller depending on the amount of greenhouse gases. emitted by human activities during the 21st century, due to the inertia of global climate systems.

Some scientific articles show that 1998 was the hottest year in the history of meteorology, that warming accelerates – 0.8 °C in a century, including 0.6 °C over the last thirty years – but also from the analysis of marine sediments, that the current heat is at the top of the temperature scale since the beginning of the Holocene, that is to say, 12,000 years ago. In February 2014, a comparison of average global temperature data, as measured by four teams, indicates 2010 as the hottest year, followed by 2005; third place is played between 1998 and 2007; 2013 is ranked 4th for one team, ranked 5th for another.

A study published in February 2013 in Global Environmental Change shows that most of the IPCC forecasts have been too optimistic (with the exception of those concerning rising temperatures): the rise in ocean levels over the period 1993- 2011 was 3.2 ± 0.5 mm/yr, 60% faster than the IPCC estimate of 2 mm/yr; the melting of the Arctic sea ice has been much faster than the worst IPCC forecasts; the increase in emissions from emerging countries has been largely underestimated (3 to 4% per year in China in the IPCC’s assumptions, 10 to 11% in reality), so that global CO₂ emissions are aligned with the most pessimistic of IPCC scenarios; amplification of warming caused by melting of permafrost has not been considered in IPCC models, etc .; the study attributes this ESLD syndrome (Erring on the Side of Least Drama) to an excessive reaction to alarmist accusations by climate-skeptics, as well as to the culture of scientific prudence.

The Global Carbon Project study, published on September 21, 2014, ahead of the UN climate summit, announces that CO₂ emissions are expected to reach 37 Gt (gigatonne = billion tonnes) in 2014 and 43.2 Gt in 2014. 2019; in 2013, they had increased by 2.3% to 36.1 Gt. In 2013, a Chinese now emits more than a European, with 7.2 tons of CO₂ per head against 6.8 tons per head in the European Union, but an American emits 16.4 tons of CO₂; the growth of these emissions is very fast in China (+ 4.2% in 2013) and in India (5.1%) while in Europe they are declining (-1.8%). The Global Carbon Project points out that the current trajectory of carbon dioxide emissions is consistent with the worst-case scenario evoked by the IPCC, which is projecting a rise in global temperature of 3.2 to 5.4 °C by 2100.

A study published on July 31, 2017 in the journal Nature Climate Change estimates at 5% the probability of limiting global warming to 2 °C by 2100, the target set by the Paris agreement on climate of 2015; the chances of achieving the 1.5 °C target, also contained in the text of the agreement, are only 1%; their forecasts, which include efforts to limit the use of fossil fuels, estimate the increase in temperature by 2100 between 2 °C and 4.9 °C, with a median value of 3.2 °C.