Climate change

The earths climate has remained relatively stable for most of human history.  This is evident from temperature records some of which date back to the mid-1800s and before this based on reconstruction from other records and measurements e.g. tree rings.   Since about the middle of the 20th Century an increasing trend is evident in the global temperature record. This is frequently referred to as global warming, but the term can include other changes such as melting of ice and thermal expansion of the oceans which is a major contributor to sea-level rise.

The greenhouse effect is the name given to the process whereby the energy which has arrived from the Sun can warm the planet to temperatures well above those which would be expected if there was no atmosphere present. A very simplified explanation of how this works is as follows:

The Sun’s energy is either reflected back into space or passes through the atmosphere to be absorbed at the surface of the Earth. The absorbed energy warms the surface, producing thermal radiation, heating the atmosphere from below.   On its way back to space, the thermal energy is captured by certain gases in the atmosphere, and this heats the atmosphere further. These gases are called greenhouse gases. This simple explanation is illustrated in the image below.

 Greenhouse gases (ghg), although they constitute only a small fraction of the atmosphere, their impact on climate is very important. Without greenhouse gases, the Earth’s surface would have an average temperature of -18oC, too cold for life as we know it. Instead the average temperature at the surface of the Earth is approximately +15oC.  Changing the atmospheric concentration of these greenhouse gases and other particles in the atmosphere can lead to a warming or cooling of the climate system. 

The IPCC provides the detailed explanation of the Greenhouse Effect.

For more information on this topic, see Chapter 2: Observations: Atmosphere and Surface, of the IPCC 5th Assessment Report, Working Group 1, The Physical Science Basis (2013)

Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, observed warming, and our current understanding of the climate system. 

It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. 

Since the start of the industrial era (about 1750), the overall effect of human activities on climate has been a warming influence. The human impact on climate during this era greatly exceeds that due to known changes in natural processes, such as solar changes and volcanic eruptions. 

For more information and further reading on this topic go IPPC Working Group 1 Annual Review 5 Summary for Policy Makers

The increases in atmospheric carbon dioxide (CO2) and other greenhouse gases during the industrial era are caused by human activities. In all cases, atmospheric concentrations of greenhouse gases, and their increases, are determined by the balance between sources (emissions of the gas from human activities and natural systems) and sinks (the removal of the gas from the atmosphere by conversion to a different chemical compound). 

Fossil fuel combustion (plus a smaller contribution from cement manufacture) is responsible for more than 75% of human-caused CO2 emissions. Land use change (primarily deforestation) is responsible for the remainder. For methane, agriculture and extraction of fossil fuels are the main sources. 

For nitrous oxide, emissions are generated by industrial and agricultural activities.  Most of the long-lived halogen-containing gases (such as chloro-fluorocarbons) are manufactured by humans, and were not present in the atmosphere before the industrial era. 

Above taken from Chapter 7, IPCC 4th Assessment Report, Working Group 1, The Physical Science Basis (2007)

Further information

Chapter 6, IPCC 5th Assessment Report, Working Group 1, The Physical Science Basis (2013)

Mitigation in climate change is a human intervention to reduce emissions or enhance the sinks of greenhouse gases. In climate policy, mitigation measures are technologies, processes or practices that contribute to mitigation, for example renewable energy (RE) technologies, waste minimization processes, public transport commuting practices.  

While mitigation measures to reduce greenhouse gas emissions have a certain cost they also constitute an economic benefit by reducing the impacts of climate change, and the costs associated with them. In addition, they may also bring economic benefits by reducing local air pollution and energy resource depletion. 

Mitigation measures can also have other benefits for society, by not only improving human health, food security, biodiversity, local environmental quality, energy access, livelihoods, and equitable sustainable development but also providing cost savings from the negative impacts of climate change as well.

Further Information

IPCC SR15 Approval Glossary

IPCC Working Group 3 Annual Report 5 Annex

IPCC Working Group 3 Annual Report 5 Summary for Policymakers