The Greenhouse Effect

The Greenhouse effect makes our planet habitable. Without it the average temperature on Earth’s surface would be about −18°C, not the pleasant 15°C we currently have. As depicted in this NASA video, greenhouse gases in the atmosphere absorb heat and reflect it back in all direction, warming the Earth. This process is called the Greenhouse Effect.

NASAs animation explaining the greenhouse effect.

Greenhouse gases (GHGs)

Water vapor, Carbon dioxide (CO2), methane(CH4), Nitrous oxide(N2O) and man made fluorinated gases Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HCFCs) are all greenhouse gases. Once released in the atmosphere each of them has a different lifetime and a different Global Warming Potential (GWP).

The global warming potential of human generated greenhouse gasses.

According to the IPCC(Intergovernmental Panel on climate change) “Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. In this report radiative forcing values are for changes relative to preindustrial conditions defined at 1750 and are expressed in Watts per square meter (W/m2).”

Radiative forcing for diffrent atmospheric compounds found in the atmosphere, natural and man made.
Radiative forcing estimates in 2011 relative to 1750 and aggregated uncertainties for the main drivers of climate change. Values are global average radiative forcing (RF14), partitioned according to the emitted compounds or processes that result in a combination of drivers. The best estimates of the net radiative forcing are shown as black diamonds with corresponding uncertainty intervals; the numerical values are provided on the right of the figure, together with the confidence level in the net forcing (VH – very high, H – high, M – medium, L – low, VL – very low). Albedo forcing due to black carbon on snow and ice is included in the black carbon aerosol bar. Small forcings due to contrails (0.05 W m–2, including contrail induced cirrus), and HFCs, PFCs and SF6 (total 0.03 W m–2) are not shown. Concentration-based RFs for gases can be obtained by summing the like-coloured bars. Volcanic forcing is not included as its episodic nature makes is difficult to compare to other forcing mechanisms. Total anthropogenic radiative forcing is provided for three different years relative to 1750.
Source: IPCC

Rise of the amount of GHGs in the atmosphere

Rising of the carbon dioxide level in the atmosphere in the last centuries.

In the past 400,000 years, during ice ages, CO2 levels were around 200 parts per million (ppm), and during the warmer interglacial periods, they hovered around 280 ppm (see fluctuations in the graph). Before the Industrial Revolution in the 19th century, global average carbon dioxide was about 280 ppm. In May 2019 sensors in Hawaii recorded Earth’s atmospheric concentration of carbon dioxide (CO2) passing 415 ppm for the first time since before the ancient dawn of humanity.

Steadily increasing concentrations of carbon dioxide in the atmosphere observed at NOAA's Mauna Loa Observatory since 1959.
A chart showing the steadily increasing concentrations of carbon dioxide in the atmosphere (in parts per million) observed at NOAA’s Mauna Loa Observatory over the course of 60 years. Measurements of the greenhouse gas began in 1959. (NOAA)

Not only CO2 emissions have risen in the last centuries. The amount of other greenhouse gases like Methane, Nitrous oxide and man made fluorinated gases has also increased in the atmosphere.

Total annual anthropogenic greenhouse gas emissions for the period 1970 to 2010 by gases.
Total annual anthropogenic greenhouse gas (GHG) emissions (gigatonne of CO2-equivalent per year, GtCO2-eq/yr) for the period 1970 to 2010 by gases: CO2 from fossil fuel combustion and industrial processes; CO2 from Forestry and Other Land Use (FOLU); methane (CH4); nitrous oxide (N2O); fluorinated gases covered under the Kyoto Protocol (F-gases).
Source: IPCC 2014(AR5)

Human activities are the main reason why the amounts of greenhouse gases in the atmosphere have increased in the last centuries.