This article examines the background assumptions for the latest global temperature projections, in an attempt to assess how global warming is likely to develop and over what time span.
The temperature of the Earth can be raised or lowered by four different factors. Known as “radiative forcing factors”, or climate forcings, they include: (1) The amount of sunlight received or reflected back into space (albedo factor). (2) Variations in Earth’s solar orbit, known as Milankovitch cycles. (3) The quantity and type of aerosol particles in the atmosphere, either from volcanic activity or human industries. (4) The levels of greenhouse gases (GHGs) in the atmosphere. For more on the planet’s heat intake, see also: Earth’s Energy Balance.
Of these, the only factor that has changed significantly in the last 100 years is the quantity of human-produced GHGs in the atmosphere, of which roughly 76 percent is carbon dioxide (CO2). The CO2 acts as a blanket, absorbing heat given off by the Earth and radiating it back down to the surface. This warming mechanism is called the “greenhouse effect.” Since 1900, CO2 and other GHGs have raised the earth’s temperature by almost 1 degree Celsius. 1
According to the Intergovernmental Panel on Climate Change (IPCC), global warming is 95 percent likely to be caused by man-made emissions of GHGs.  The European Commission estimates that 87 percent of man-made climate change is caused by just three greenhouse gases: carbon dioxide (the cause of 64 percent warming), methane (17 percent) and nitrous oxide (6 percent). 2
- What Are The Latest Figures On Greenhouse Gases?
- Fossil Fuels Cause 76 Percent Of Greenhouse Gas Emissions
- How Much Has Earth’s Temperature Risen?
- How Hot Is Earth Going To Get By 2100?
- What Exactly Do Climate Scientists Predict About Future Warming?
- Global Temperature Projections To 2100
- Expected GHG Emission Levels Versus Benchmark Levels
- The Emissions Gap
- Deep Contradictions In U.S. Climate Action Policy
- How Hot Will The World Get? A Summary of Global Temperature Projections
What Are The Latest Figures On Greenhouse Gases?
The latest statistics on global greenhouse gas emissions are as follows: total emissions for the year 2018 amounted to 55.3 GtCO2e (billion tons of CO2 equivalent). Of these, 37.5 GtCO2 came from fossil fuels and industry; 5.13 GtCO2 came from deforestation and land use; 8.8 GtCO2e came from man-made methane emissions; 3 GtCO2e came from man-made nitrous oxide; 1 GtCO2e came from fluorinated gases. [Note: like many greenhouse gas statistics, these are approximate figures only. Sources include: “Emissions Gap Report 2019.” UNEP; “Greenhouse Gas Concentrations Report” World Meteorological Organization (2019); “Global Carbon Budget 2019”; and other institutions like the United Nations (UN).]
Fossil Fuels Cause 76 Percent Of Greenhouse Gas Emissions
More than three quarters of greenhouse gas pollution – at least in the USA – is caused by the composition of fossil fuels like petroleum (oil), coal, and natural gas – largely in power plants and transportation. 3 Now that the EU and the U.S. EPA have reclassified wood burning as carbon neutral, we face additional carbon emissions and the potential loss of important forests (carbon sinks) around the world. This merely adds to global warming.
The Earth’s average surface temperature is 15°C (59°F) and rising. By comparison, the average temperature on Mars is about minus 60°C (minus 80°F) and on Venus a boiling hot 462°C (864°F).
How Much Has Earth’s Temperature Risen?
Since 1900, CO2 and other greenhouse gases have helped to raise the earth’s temperature by more than 1 degree Celsius.
According to NASA’s Goddard Institute for Space Studies, the planet’s average surface temperature has increased by roughly 1.1 degrees Celsius (2.0 degrees Fahrenheit) since the late 19th century. 4 (See also: When Did Global Warming Start?)
How Hot Is Earth Going To Get By 2100?
Scientists estimate that, by 2100, the average temperature of the planet will rise between 0.3°C and 4.8°C (0.5°F to 8.6°F) above the pre-industrial baseline (1850-1800). 5 The actual outcome will be determined by the scale and commitment of the global response to the climate crisis and how quickly emissions can be reduced. (See also: Rising Temperatures on Earth: What to Expect.)
What Exactly Do Climate Scientists Predict About Future Warming?
The most authoritative reports on the latest climate science, including global temperature projections, are compiled every six years by the IPCC. In their Fifth Assessment Report (2014), the IPCC selected four possible scenarios for how global warming might develop, called “representative concentration pathways” (RCPs).
IPCC’s 4 Different Scenarios or Pathways
These representative concentration pathways” (RCPs) are: RCP2.6 – RCP4.5 – RCP6 – and RCP8.5. The numbers in each RCP refer to the amount of radiative forcing (in watts per square metre), which is associated with each particular scenario at the end of the century. For example, in RCP2.6 the radiative forcing is 2.6 watts per square metre in 2100.
Each RCP represents a specific scenario based on a set of assumptions concerning greenhouse gas emissions up to 2100, as well as assumptions about: carbon capture and storage technologies, consumption of renewable energy, amount of deforestation and reforestation, investment in climate technology, and so on. In addition, each RCP includes differing data on economic activity, energy sources, population growth and other socio-economic factors.
How Do The RCPs Compare?
- RCP 8.5 was produced using the MESSAGE model and the IIASA Integrated Assessment Framework by the International Institute for Applied Systems Analysis in Austria. RCP8.5 shows what happens when there is no global plan of climate action. It leads to very high greenhouse gas emissions resulting in CO2 concentration levels of 940 parts per million (ppm) by 2100. RCP8.5 leads to global warming of between 2.6°C and 4.8°C (possibly as high as 5.5°C), with potentially catastrophic effects for the Earth’s climate system and ecosphere.
- RCP 6.0 was created by the AIM modelling team at the National Institute for Environmental Studies (NIES) in Japan. It demonstrates an intermediate scenario, in which emissions peak around 2080, then decline. Emission reductions are achieved through the application of some mitigation strategies and technologies. CO2 concentration rises less rapidly than RCP8.5, but still reaches 660 ppm by 2100 before stabilizing shortly afterwards. By 2100, RCP6.0 is projected to lead to global warming of between 1.4°C and 3.1°C.
- RCP 4.5 was developed by the GCAM modelling team at the Pacific Northwest National Laboratory’s Joint Global Change Research Institute in the USA. Like RCP6.0 it represents an intermediate scenario, in which emissions peak around 2040, then decline. Curiously, concentrations of CO2 are slightly higher than those of RCP6.0 until after mid-century, but emissions peak earlier (around 2040), and CO2 concentration reaches 540 ppm by 2100. RCP4.5 leads to warming of between 1.1°C and 2.6°C.
- RCP 2.6 was created by the IMAGE modelling team of the PBL Netherlands Environmental Assessment Agency. It is the most stringent of all the scenarios, and aims to keep global warming below 2°C. In doing so, it assumes that annual GHG emissions reach their peak around 2020, after which they decline rapidly. CO2 concentrations peak at about 440 ppm around 2050, followed by a slow decline to around 400 ppm by the end of the century. The success of this scenario is contingent upon global cooperation among all CO2 emitters – including developing countries – as well as the application of CDR and carbon capture and storage (CCS) technologies. RCP leads to warming of between 0.3°C and 1.7°C.
Fossil Fuel Use Balanced By CCS Technologies
Fossil-fuel consumption is generally in line with the radiative forcing level of each RCP scenario. However, by employing carbon capture and storage (CCS) technologies (notably in the power sector), all scenarios, by 2100, are still using more coal and/or natural gas than in 2000. In fact, oil consumption remains fairly constant in all scenarios, except RCP2.6. The use of non-fossil fuels increases in all four scenarios, especially renewable sources like wind, solar and bio-energy, as well as nuclear power. Not surprisingly, the main stimulants are rising energy demand, rising fossil-fuel prices and a more stringent climate policy. In RCP2.6 the increased use of bio-energy and CCS is especially important in boosting negative emissions. 6
Warming To Exceed 1.5°C In All Scenarios Says AR5
The IPCC’s 2014 Fifth Assessment Report concluded that by 2100, global temperature projections are likely to exceed 1.5°C in all scenarios except RCP2.6. The median rise in temperature for each RCP is: 3.7 degrees Celsius (RCP8.5) – 2.2 degrees (RCP6.0) – 1.8 degrees (RCP4.5) – and 1 degree (RCP2.6). According to these figures, both of the lower scenarios produce warming of less than 2°C, which seems low. Are these projections credible? Let’s see.
Paris Agreement Limits Warming To “Well Below 2 Degrees”
Based on the IPCC’s 2014 Report, the Paris Climate Agreement (2015) produced a framework for international climate action that set the target for global warming at “well below 2°C”, while at the same time urging countries to adopt the even lower target of 1.5°C. How did it propose to achieve either of these targets? Answer: By persuading a large number of countries (186 at the last count) to sign up to a system of pledges – called “nationally determined contributions” (NDCs) – that would (when finalized) meet the target. It was also agreed that new NDCs (of increasing stringency) should be submitted every five years. The delegates cheered and then everyone went home. Did the Paris Agreement solve the world’s climate crisis? The IPCC didn’t think so.
First up was Professor David G Victor, Chairman of the Global Agenda Council on Governance for Sustainability at the World Economic Forum. As soon as the Paris conference was over, he stated than the 2015 Paris Agreement goals on climate change were unsustainable. “The world has dithered for too long and must now brace for the consequences. Even a realistic crash program to cut emissions will blow through 2 degrees; 1.5 degrees is ridiculous.” For the preceding five years Professor Victor was on the Intergovernmental Panel on Climate Change (IPCC) helping to assess what reductions in greenhouse gases are feasible, so his opinion is important. 7
Three years later, the IPCC delivered its considered opinion on the conclusions reached at Paris.
Reality Check – IPCC’s Special Report On Global Warming
The IPCC’s 2018 Special Report on Global Warming of 1.5°C (SR15) was a reality check for world leaders. It stated that even if all parties to the Paris Agreement implemented their NDCs in full, net greenhouse gas emissions would increase, compared to 2010. 5 And if warming continued at present rates, global temperatures would likely rise to 1.5°C (above pre-industrial levels) between 2030 and 2052, and to 3-4°C by 2100 – a catastrophic outcome for the Earth’s biosphere. 8
Special Report Issues A Warning To Global Policy-Makers
The report warns that while meeting the 1.5°C target is feasible within the laws of physics, global emissions of CO2 (as well as methane, nitrous oxide, black carbon) would have to fall roughly 45 percent, from 2010 levels, by 2030; and then to ‘net zero’ by 2050. Even the 2°C target, will require CO2 emissions to fall by 25 percent by 2030, and to ‘net zero’ by 2075.
The report states bluntly that these unprecedented cuts in emissions will necessitate dramatic changes in energy consumption and the rapid phasing out of fossil fuels, as well as strong international cooperation. By contrast, a global response that consisted of limited international cooperation, leading to increasing or stagnating greenhouse gas emissions, would put 1.5°C out of reach.
Global Temperature Projections To 2100
Here is a visual layout, reproduced courtesy of Climate Action Tracker, of the likely results of the various emission pathways.
- In the absence of any climate action policies, emissions will continue unchecked leading to climate warming in the range of 4.1°C to 4.8°C (above 1850-1900 levels) by the year 2100. This baseline scenario is similar to the RCP 8.5 pathway outlined in the IPCC’s Fifth Assessment Report. 9
- Current policies on greenhouse gas emissions presently in place around the world are forecast to result in about 3.3°C warming (3.3°C being the mid-point of current policy predictions: 3.1 to 3.5°C).
- Emission trajectories based on unconditional pledges (NDCs) made by governments at Paris in 2015 or afterwards, lead to warming of about 3.0°C – assuming that policies of similar strength continue to be implemented until 2100.
- The optimistic emission projection comprises a scenario run by Climate Action Tracker that assumes the fulfilment of more ambitious government climate actions that are in the pipeline but not yet implemented.
- At the foot of the above diagram are the projected emission trajectories for 1.5°C and 2°C warming levels.
As we can see, current policy and pledge trajectories are significantly above pathways consistent with the Paris Agreement’s long-term global temperature projections of “well below 2°C”.
Expected GHG Emission Levels Versus Benchmark Levels
The diagram below compares the expected absolute GHG emissions in 2020, 2025, and 2030 with benchmark emissions needed to achieve the 1.5°C target reached in Paris and – for reference purposes – with those projected for the former 2°C Cancun goal.
(Note: In 2010, signatories to the UNFCCC adopted the Cancun Agreements which acknowledged the need for deep cuts in greenhouse gas emissions in order to limit global temperature projections to 2°C above 1850-1900 levels. This is referred to as the “Cancun 2°C goal”.)
The Emissions Gap
As of December 2018, there is still a significant emissions gap between the greenhouse gas emission levels projected in the NDCs submitted under the Paris Agreement, and the levels that will be needed to achieve the global temperature goals of the Paris Agreement (1.5°C and 2°C).
(A) EMISSIONS TO ACHIEVE 1.5°C TARGET
41 GtCO2e 10 in 2025 and 28 GtCO2e in 2030.
(B) EMISSIONS FORECAST IN NDCs
53–55 GtCO2e in 2025 and 54–57 GtCO2e in 2030.
12–14 GtCO2e in 2025, 26–29 GtCO2e in 2030.
(A) EMISSIONS TO ACHIEVE 2°C TARGET
47 GtCO2e in 2025 and 39 GtCO2e in 2030.
(B) EMISSIONS FORECAST IN NDCs
53–55 GtCO2e in 2025 and 54–57 GtCO2e in 2030.
6–8 GtCO2e for 2025 and 15–18 GtCO2e in 2030
Progress Report On Government Climate Action
- As of June 2019, despite some positive announcements and developments, little action is being taken towards achieving the Paris Climate Agreement’s long-term target of keeping global warming “well below 2°C”, while pushing for the lower goal of 1.5°C. And all the while we continue to heat up the atmosphere.
- Greenhouse gas emissions continue to increase across the world. In 2018, man-made emissions reached a historic high of 33.1 GtCO2, of which more than 33 percent were from coal 11 The United States, India and China alone were responsible for 85 percent of the rise in emissions.
- Sources of renewable energy are stagnating after 20 years of strong growth. The year 2018 was the first since 2001 to see no growth in capacity for renewable energy. 11
Why are governments and corporations so slow at taking effective climate action? For the answer, see: Root Cause of Climate Change.
Irreversible Tipping Points May Be Looming
True, a few countries (Argentina, Britain, Canada, Chile, Costa Rica, Ethiopia, the EU, India and Morocco) are taking steps in the right direction by (e.g.) discussing ‘net zero’ emissions targets, but most governments are still sleepwalking towards the abyss, with seemingly no idea that irreversible tipping points may be only a few years away. The truth is, radical climate change mitigation policies by all governments are needed now, supported by proper international cooperation, intensive planning and climate investment, as well as significant transfers of technology and other resources to developing nations. We need to cut global emissions in half by 2030 in order to hit 1.5°C by the end of the century.
Deep Contradictions In U.S. Climate Action Policy
In a recent 500-page environmental impact statement concerning the possible effect on greenhouse gas levels of a Presidential plan to deregulate car exhaust emissions, the U.S. National Highway Traffic Safety Administration (NHTSA) projected that, by 2100, global temperature will rise by 7 degrees Fahrenheit (roughly 4 degrees Celsius) above the average temperature of the period 1986-2005.
If these global temperature projections are true, such a rise would be catastrophic for humans as well as the planet’s entire ecological system. In addition, parts of Manhattan and Miami would be swamped by the ocean, let alone Pacific islands and low-lying areas of the Indian subcontinent. But instead of saying more on this grim subject – or perhaps revealing how the Administration planned to deal with this impending disaster – the report simply moved on to say that although the deregulation proposal would increase CO2 emissions, the new policy would add hardly anything to what was already a huge mess.
In fact, according to the government’s own analysis, the car exhaust proposal will put 8 billion additional tons of carbon dioxide into the atmosphere this century: more than the total of U.S. emissions for an entire year.
The report did not attempt to reconcile its prediction of climate disaster with President Trump’s well known opinion that climate change is a hoax, or his recent attempts to dismantle nearly half a dozen major rules aimed at reducing greenhouse gases.
On reading the report, Michael MacCracken, a senior scientist at the U.S. Global Change Research Program from 1993 to 2002, couldn’t believe it: “They’re saying that human activities are going to lead to this rise of carbon dioxide that is disastrous for the environment and society. And then they’re saying they’re not going to do anything about it” he said. 12
How Hot Will The World Get? A Summary of Global Temperature Projections
- Between 4.8°C and 5.7°C if nothing is done. (IPCC Special Report 2018)
- 4°C (7°F) (U.S. National Highway Traffic Safety Administration. August 2018)
- Between 3.1°C and 3.5°C assuming current policies continue. (Climate Action Tracker. Dec 2018)
- 3°C if all Paris Agreement NDCs are fulfilled continually until 2100. (IPCC Special Report 2018)
- 2°C if the world cooperates in a well-financed, comprehensive, climate action plan. (IPCC Special Report 2018)
- 1.5°C? I don’t think so. Not unless the world changes.
- IPCC Special Report on Global Warming of 1.5°C: Summary for policymakers (1)
- EU Commission. (2)
- Where Greenhouse Gases Come From. U.S.EIA. (3)
- The Globe Is Already Above 1°C, on Its Way to 1.5°C. Climate Central. (4)
- IPCC Synthesis Report (5)
- The representative concentration pathways: an overview. Van Vuuren, D.P., Edmonds, J., Kainuma, M. et al. Climatic Change 109, 5 (2011). (6)
- “Why Paris Worked: A Different Approach to Climate Diplomacy,” David G Victor. (7)
- “IPCC issues Special Report on Global Warming of 1.5°C.” World Meteorological Organization (WMO). (8)
- Climate Action Tracker. “2100 Warming Projections.” December 2018. The Climate Action Tracker (CAT) is an independent scientific analysis that compares government climate action against the globally agreed target of holding climate warming “well below 2°C” (Paris Agreement 2015), while pursuing the more ambitious target of 1.5°C. CAT is produced by three research organisations: Climate Analytics, Ecofys and NewClimate Institute. (9)
- Gigatonnes of equivalent carbon dioxide.(10)
- International Energy Agency (IEA). 2019. (11)
- “Trump administration sees a 7-degree rise in global temperatures by 2100.” Juliet Eilperin, Brady Dennis, Chris Mooney. Washington Post. September 28, 2018. (12)