Greenhouse gases
Greenhouse gases include carbon dioxide and methane.
Greenhouse gas emissions
Humans have drastically increased greenhouse gas concentrations in the atmosphere over the last 200 years. These are called anthropogenic emissions and are the leading cause of climate change.
How greenhouse gases impact Earth's atmosphere
Without greenhouse gases Earth's average temperature would be roughly the same as the moon, i.e. -18°C., and it'd swing wildly between -170°C and 100°C across a day. During the day our atmosphere blocks out the most harmful rays and about 1/3 of visible light, and at night it traps infra-red radiation to keep the temperature higher.
Only gases with electric charges (i.e. more electrons than protons) block solar energy. Most of the atmosphere is made of molecules, like oxygen, nitrogen, and water, that don't have an electric charge (i.e. balanced number of electrons and protons). However some molecules are "lopsided" (i.e. most protons on one side and electrons on the other side of the molecule) and can absorb the energy of infra-red rays - water (H2O), ozone (O3), and nitrous oxide (N2O) are examples.
Carbon dioxide and methane are examples of gases that internally bounce around enough to sometimes become "lopsided" and also absorb infra-red rays. The vast majority of our atmosphere (oxygen and nitrogen) cannot absorb infra-red rays. Despite this, the gases that can absorb this energy can absorb up to 90% of outgoing heat, despite only making up 1% of the atmosphere. These are greenhouse gases. Greenhouse gases trap heat energy leaving Earth's surface and radiate it back towards Earth. Records from ice show that small variations in carbon dioxide produce big variations in temperature.
Comparing greenhouse gas emissions
Global Warming Potential (GWP)
Global Warming Potential is a measurement that combines the lifetime and radiative efficiency of a greenhouse gas. The IPCC defines a GWP as "the time-integrated radiative forcing due to a pulse emission of a given component, relative to a pulse emission of an equal mass of CO2."
GWP is denoted as CO2e, or carbon dioxide equivalent.
GWP-100 is the most commonly used metric. It refers to the global warming potential of a GHG over 100 years.
GWP measurements can help to compare reduction opportunities across gases and sectors. However it is an imperfect measurement - for example it undercuts the temperature impact methane emissions can have in the short-term. There are alternatives metrics like GWP* and the combined-GTP (CGTP) that seek to make up for this shortcomings, but they too have critics.
Between sectors
Many emissions are associated with various energy-related sectors—electricity generation, energy for transportation, building operations, etc. But defining the "boundary" around a sector can be subjective, and can really influence our perception of what "matters" when it comes to emissions reductions, and what really impactful solutions are possible.
Take, as an example, the global food system. One common metric is that "agriculture and associated land-use change account for 24% of total global emissions." However, when you include transportation, processing, and fertiliser manufacturing associated with the food system, the global food system may contribute up to 34 percent of global GHG emissions. Which sector are these emissions in? Transportation, manufacturing, food, or some mix? When you take a more integrated view, "food systems"—not just agricultural production—should be a critical focus for greenhouse gas mitigation and adaptation strategies.
This focal shift offers a greater mitigation potential by finding ways to improve efficiency and minimise tradeoffs among food system actions. It also allows us to integrate mitigation activities that may fall outside "agricultural production" (e.g. reducing food waste/landfill of food items, shifting more towards improved distribution and composting, and reducing packaging/processing).
Halocarbons
Halocarbons are particular potent when it comes to greenhouse gas emissions. They are mainly used as refrigerants.
Emissions by sector
Energy
Energy consumption is by far the biggest source of human-caused greenhouse gas emissions, responsible for a whopping 75.6% (37.6 GtCO2e) worldwide. The energy sector includes transportation, electricity and heat, buildings, manufacturing and construction, fugitive emissions and other fuel combustion. 1
Meeting climate targets requires reducing fossil fuel's share of our global energy system by about 2-3 percentage points per year. Last year's change was +0.3 percentage points. 😠 We aren't even moving the right direction. 2
Electricity
61% of global energy comes from fossil fuels (as of 2022).
Transportation
Road transportation made up 12.6 percent of 2019 GHG emissions.