Climate Forcing

2023-03-21

Table of contents

Climate Forcing
Climate Change Indicators: Climate Forcing
Key Points
Background
About the Indicator
About the Data
- Indicator Notes

Climate Forcing

Climate forcing is any factor that influences the Earth’s climate system and drives changes in the global climate. These factors can be natural or human-induced and can positively and negatively impact the Earth’s atmosphere.

Some examples of natural climate forcings include changes in solar radiation, volcanic eruptions, and variations in the Earth’s orbit around the sun. On the other hand, human-induced climate forcings have the burning of fossil fuels, deforestation, and releasing greenhouse gases into the atmosphere.

Climate forcing can affect the Earth’s climate by altering the balance of energy in the Earth’s atmosphere and oceans. For example, greenhouse gases, such as carbon dioxide and methane, trap heat in the Earth’s atmosphere, causing global temperatures to rise. This is known as the greenhouse effect, one of the main drivers of climate change.

It is essential to understand the different factors that contribute to climate forcing, as this knowledge can help us better understand how the Earth’s climate is changing and how we can mitigate the negative impacts of climate change.

Climate Change Indicators: Climate Forcing

This indicator measures the “radiative forcing” or heating effect caused by greenhouse gases in the atmosphere.

Figure 1. Radiative Forcing Caused by Major Long-Lived Greenhouse Gases, 1979-2019

This figure shows the amount of radiative forcing caused by various greenhouse gases due to the change in the concentration of these gases in the Earth’s atmosphere since 1750. The radiative forcing is calculated in watts per square meter, representing the magnitude of the energy imbalance in the atmosphere. On the right side of the graph, radiative forcing has been converted to the Annual GHG Index for 1990, set to 1.0.

Year	Carbon dioxide	Methane	Nitrous oxide	CFC-12	CFC-11	15 other gases
1979	1.027	406	104	92	0.04	31
1980	1.058	413	104	97	42	34
1981	1.077	0.42	107	102	44	36
1982	1.089	426	111	107	46	38
1983	1.115	429	113	113	48	41
1984	1.14	432	116	118	0.05	44
1985	1.162	437	118	123	53	47
1986	1.184	442	121	129	56	49
1987	1.211	447	0.12	135	58	53
1988	1.25	451	122	143	61	57
1989	1.275	455	126	149	63	61
1990	1.293	459	129	154	65	65
1991	1.312	463	131	158	66	69
1992	1.323	467	133	162	67	72
1993	1.334	467	134	164	67	74
1994	1.356	0.47	135	165	67	76
1995	1.383	472	136	168	67	77
1996	1.41	473	139	0.17	66	78
1997	1.426	474	142	171	66	79
1998	1.464	478	144	172	66	0.08
1999	1.495	481	148	173	65	82
2000	1.513	481	151	173	65	83
2001	1.535	0.48	153	174	64	85
2002	1.564	481	155	174	64	87
2003	1.6	483	157	174	63	89
2004	1.627	483	159	174	63	0.09
2005	1.655	482	162	173	62	92
2006	1.685	482	165	173	62	95
2007	1.71	484	167	172	61	98
2008	1.739	486	0.17	171	61	0.1
2009	1.76	489	172	171	0.06	103
2010	1.791	491	175	169	0.06	106
2011	1.817	492	178	169	59	109
2012	1.845	494	181	168	59	112
2013	1.882	496	183	167	58	114
2014	1.908	499	187	166	58	117
2015	1.938	504	0.19	165	58	119
2016	1.985	507	193	164	57	122
2017	2.013	509	195	163	57	124
2018	2.044	512	199	162	57	127
2019	2.076	516	202	161	57	129

Figure 1. Radiative Forcing Caused by Major Long-Lived Greenhouse Gases, 1979-2019
Data source: NOAA, 2020
Web update: April 2021
Units: Watts per square meter

Figure 2. Radiative Forcing Caused by Human Activities Since 1750

This figure represents the total amount of radiative forcing caused by human activities, including indirect effects, between 1750 and 2011. The radiative forcing is calculated in watts per square meter, representing the magnitude of the energy imbalance in the atmosphere. Each colored bar represents the scientists’ best guess, while thin black bars show the range of possible probabilities. The natural variation in energy received from the sun during this period is given as a reference.

Key Points

The Annual GHG Index in 2019 was 1.45, representing a 45 percent increase in radiative forcing since 1990 (see Figure 1).
Among the greenhouse gases shown in Figure 1, carbon dioxide has by far the largest share of radiative forcing since 1990, and its contribution continues to increase at a steady rate. Carbon dioxide alone accounts for a 36 percent increase in radiation forcing since 1990.
Although the overall Annual GHG Index continues to rise, the rate of increase has slowed slightly since the reference year 1990. This change was primarily due to methane gas. As the production of CFCs has been phased out globally due to their damage to the ozone layer, concentrations have increased at a slower rate in recent years, and chlorofluorocarbon concentrations are decreasing (see Figure 1).
Greenhouse gases produced by human activities have caused an overall warming effect on Earth’s climate since 1750. Carbon dioxide contributed the most to warming, followed by methane and black carbon. Although aerosol pollution and other activities cause cooling, the net result is that human activities are warming the Earth (see Figure 2).

Background

When energy from the sun reaches Earth, the planet absorbs some of that energy and radiates the rest back into space as heat. The earth’s surface temperature depends on this balance between incoming and outgoing energy. Average conditions tend to remain constant unless the Earth experiences a force that changes its energy balance. A change in energy balance causes the Earth’s average temperature to become warmer or colder, leading to various other changes in the lower atmosphere, land, and oceans.

Various physical and chemical changes can affect the global energy balance and cause changes in the Earth’s climate. While some of these changes are natural, some of them are human-induced. These changes are measured by the amount of warming or cooling they can produce, called “radial forcing.” Differences with a warming effect are called “positive” forcing, and changes with a cooling effect are called “negative.” When positive and negative forces are out of balance, the result is a change in Earth’s average surface temperature.

Changes in greenhouse gas concentrations in the atmosphere affect radiative forcing. Greenhouse gases absorb the energy from the Earth’s surface, releasing the heat back into the lower atmosphere and warming the Earth’s surface. Human activities have caused increased concentrations of greenhouse gases in the atmosphere, which can remain for decades, centuries, or longer, so the corresponding warming effects will persist for a long time.

About the Indicator

Figure 1 of this indicator measures the average total radiative forcing of 20 long-lived greenhouse gases, including carbon dioxide, methane, and nitrous oxide. The results were calculated by the National Oceanic and Atmospheric Administration based on measured concentrations of gases in the atmosphere compared to concentrations present around 1750 before the Industrial Revolution began.

Since each gas has a different ability to absorb and emit energy, this indicator converts changes in greenhouse gas concentrations into a measure of the total radiative forcing caused by each gas. The radiative forcing is calculated in watts per square meter, representing the magnitude of the energy imbalance in the atmosphere.

NOAA also translates the total radiative forcing of these measured gases into an index value called the Annual Greenhouse Gas Index (right side of Figure 1). This number compares radiative forcing for a given year with radiative forcing in 1990, a typical base year for global agreements to monitor and reduce greenhouse gas emissions.

This indicator also estimates the total radiative forcing associated with various human activities from 1750. Figure 2 shows the influence of:

Tropospheric ozone is a short-lived greenhouse gas.
Emissions that indirectly cause greenhouse gases through chemical reactions in the atmosphere. For example, methane emissions also cause an increase in tropospheric ozone.
Aerosol pollution consists of suspended solid and liquid particles that can reflect incoming sunlight.
Black carbon, which, when deposited on snow and ice, can make the Earth’s surface darker and less reflective.
Other factors, such as land use change, affect radiative forcing.

About the Data

Indicator Notes

The index in Figure 1 does not include short-lived greenhouse gases such as tropospheric ozone, reflective aerosol particles, black carbon, or the indirect effect of methane through its impact on water vapor and ozone formation. Figure 2 includes these and other indirect effects.