guest post by Steve Easterbrook
(2) Humans caused the majority of it
The summary for policymakers says:
It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.
This chart summarizes the impact of different drivers of warming and/or cooling, by showing the total cumulative energy added to the earth system since 1970 from each driver. Note that the chart is in zettajoules (1021J). For comparison, one zettajoule is about the energy that would be released from 200 million bombs of the size of the one dropped on Hiroshima. The world’s total annual global energy consumption is about 0.5 zettajoule.
Long lived greenhouse gases, such as CO2, contribute the majority of the warming (the purple line). Aerosols, such as particles of industrial pollution, block out sunlight and cause some cooling (the dark blue line), but nowhere near enough to offset the warming from greenhouse gases. Note that aerosols have the largest uncertainty bar; much of the remaining uncertainty about the likely magnitude of future climate warming is due to uncertainty about how much of the warming might be offset by aerosols. The uncertainty on the aerosols curve is, in turn, responsible for most of the uncertainty on the black line, which shows the total effect if you add up all the individual contributions.
The graph also puts into perspective some of other things that people like to blame for climate change, including changes in energy received from the sun (‘solar’), and the impact of volcanoes. Changes in the sun (shown in orange) are tiny compared to greenhouse gases, but do show a very slight warming effect. Volcanoes have a larger (cooling) effect, but it is short-lived. There were two major volcanic eruptions in this period, El Chichón in 1982 and and Pinatubo in 1992. Each can be clearly seen in the graph as an immediate cooling effect, which then tapers off after a a couple of years.
You can download all of Climate Change 2013: The Physical Science Basis here. Click below to read any part of this series:
- The warming is unequivocal.
- Humans caused the majority of it.
- The warming is largely irreversible.
- Most of the heat is going into the oceans.
- Current rates of ocean acidification are unprecedented.
- We have to choose which future we want very soon.
- To stay below 2°C of warming, the world must become carbon negative.
- To stay below 2°C of warming, most fossil fuels must stay buried in the ground.
Climate Change 2013: The Physical Science Basis is also available chapter by chapter here:
- Observations: Atmosphere and Surface
- Observations: Ocean
- Observations: Cryosphere
- Information from Paleoclimate Archives
- Carbon and Other Biogeochemical Cycles
- Clouds and Aerosols
- Anthropogenic and Natural Radiative Forcing
- Evaluation of Climate Models
- Detection and Attribution of Climate Change: from Global to Regional
- Near-term Climate Change: Projections and Predictability
- Long-term Climate Change: Projections, Commitments and Irreversibility
- Sea Level Change
- Climate Phenomena and their Relevance for Future Regional Climate Change
- Annex I: Atlas of Global and Regional Climate Projections
- Annex II: Climate System Scenario Tables
- Annex III: Glossary
- Annex IV: Acronyms
- Annex V: Contributors to the WGI Fifth Assessment Report
- Annex VI: Expert Reviewers of the WGI Fifth Assessment Report