The Earth’s Climate
Climate change is altering temperature, precipitation, and sea levels, and will adversely impact human and natural systems, including water resources, human settlements and health, ecosystems, and biodiversity. The unprecedented acceleration of climate change over the last 50 years and the increasing confidence in global climate models add to the compelling evidence that climate is being affected by greenhouse gas (GHG) emissions from human activities.2
Changes in climate should not be confused with changes in weather. Weather is observed at a particular location on a time scale of hours or days, and exhibits a high degree of variability, whereas climate is the long-term average of short-term weather patterns, such as the annual average temperature or rainfall.3 Under a stable climate, there is an energy balance between incoming short wave solar radiation and outgoing long wave infrared radiation. Solar radiation passes through the atmosphere and most is absorbed by the Earth’s surface. The surface then re-emits energy as infrared radiation, a portion of which escapes into space. Increases in the concentrations of greenhouse gases in the atmosphere reduce the amount of energy the Earth’s surface radiates to space, thus warming the planet.4
The Earth's Greenhouse Effect1
- Disturbances of the Earth’s balance of incoming and outgoing energy are referred to as positive or negative climate forcings. Positive forcings, such as GHGs, exert a warming influence on the Earth, while negative forcings, such as sulfate aerosols, exert a cooling influence.5
- Increased concentrations of GHGs from anthropogenic sources have increased the absorption of infrared radiation, enhancing the natural greenhouse effect. Methane and other GHGs are more potent, but CO2 contributes most to warming because of its prevalence.5
- Anthropogenic GHG emissions, to date, amount to a climate forcing roughly equal to 1% of the net incoming solar energy, or the energy equivalent of burning 13 million barrels of oil every minute.6
Climate Feedbacks and Inertia
- Climate change is also affected by the Earth’s responses to forcings, known as climate feedbacks. For example, the increase in water vapor that occurs with warming further increases climate forcing and evaporation, as water vapor is a powerful GHG.5
- The volume of the ocean results in large thermal inertia that slows the response of climate change to forcings; energy balance changes result in delayed climate response with high momentum.7
- As polar ice melts, less sunlight is reflected and the oceans absorb more solar radiation.5
- Due to increasing temperature, large reserves of organic matter frozen in subarctic permafrost will thaw and decay, releasing additional CO2 and methane to the atmosphere.8 June 2020 was tied for the warmest on record and extreme temperatures in the Artic (especially Siberia) contributed to large wildfires and further thawing of permafrost. The fires alone were estimated to have released 59 MMT of CO2 into the atmosphere.9
- If GHG emissions were completely eliminated today, climate change impacts would still continue for centuries.10 The Earth’s temperature requires 25 to 50 years to reach 60% of its equilibrium response.11
- Today’s emissions will affect future generations; CO2 persists in the atmosphere for hundreds of years.12
Human Influence on Climate
- Separately, neither natural forcings (e.g., volcanic activity and solar variation) nor anthropogenic forcings (e.g., GHGs and aerosols) can fully explain the warming experienced since 1850.13
- Climate models most closely match the observed temperature trend only when natural and anthropogenic forcings are considered together.13
- In 2013, the Intergovernmental Panel on Climate Change (IPCC) concluded that: “It is extremely likely (>95 % certainty) that human influence has been the dominant cause of the observed warming since the mid-20th century.”5
Modeled and Observed Global Average Temperatures14
- Global average temperature was 0.98oC (1.76oF) higher in 2020 than in the late 1800s.15
- The warmest year on record since records began in 1880 was 2016, with 2020 ranking second. In 2020 global average land temperatures experienced a record high, while 2016 global ocean temperatures remain the highest on record. The seven warmest years since 1880 have all occurred since 2014 and in 2020 annual global temperatures were above average for the 44th consecutive year.15
- Annual 2020 arctic temperatures rose to 1.9oC above the 1981-2010 average. Arctic sea ice is becoming younger, thinner, and less expansive. The 2020 extent of ice reached the second lowest annual cover on record since 1979, 3.74 million square kilometers.16
- U.S. average annual precipitation has increased by 4% since 1901, but the intensity and frequency of extreme precipitation events has increased even more, a trend that is expected to continue.17
- In the 20th century, global mean sea level rose between 17 and 21 cm, after having been quite stable over the previous several thousand years.5
- Snow cover has noticeably decreased in the Northern Hemisphere. From 1967-2012, snow cover extent decreased by approximately 53% in June, and around 7% in March and April.5
Northwestern Glacier melt, Alaska, 1940-200518
- Warming that has already occurred is affecting the biological timing (phenology) and geographic range of plant and animal communities.19 Relationships such as predator-prey interactions are affected by these shifts, especially when changes occur unevenly between species.20
- Since the start of the 20th century, the average growing season in the contiguous 48 states has lengthened by nearly two weeks.21
- By 2035, IPCC predicts that the temperature will rise between 0.3-0.7oC (0.5-1.3oF). In the long term, global mean surface temperatures are predicted to rise 0.4-2.6oC (0.7-4.7oF) from 2045-2065 and 0.3-4.8oC (0.5-8.6oF) from 2081-2100, relative to the reference period of 1986-2005. Since 1970, global average temperatures have been rising at a rate of 1.7oC per century, significantly higher than the average rate of decline of 0.01oC over the past 7,000 years.5,22
- A warming planet does not simply result in higher average daytime temperatures, the frequency and magnitude of extreme hot days will increase.22
Projected Annual Mean Change in Temperature (°C), 21st Century5
- Models anticipate sea level rise between 26 and 77 cm for a 1oC increase in temperature. The rise will be a result of thermal expansion from warming oceans and additional water added to the oceans by melting glaciers and ice sheets.22
- The oceans absorb about 27% of anthropogenic CO2 emissions, resulting in increased acidity. Even under conservative projections, coral reefs will be severely impacted.23
Implications for Human and Natural Systems
- Impacts of climate change will vary regionally but are very likely to impose costs that will increase as global temperatures increase.10
- This century, an unprecedented combination of climate change, associated disturbances, and other global change drivers will likely exceed many ecosystems’ capacities for resilience.24 Species extinction, food insecurity, human activity constraints, and limited adaptability are risks associated with warming at or above predicted temperatures for the year 2100 (4oC or 7oF above pre-industrial levels).10
- With an increase in average global temperatures of 2oC, nearly every summer would be warmer than the hottest 5% of recent summers.25
- Due to regional variation, a 2-foot rise in sea level would cause relative increases of 3.5 feet in Galveston, TX and 1 foot in Neah Bay, WA.2
- Increased temperatures, changes in precipitation and climate variability would alter the geographic ranges and seasonality of diseases spread by organisms like mosquitoes.25
- Although higher CO2 concentrations and slight temperature increases can boost crop yields, the negative effects of warming on plant health and soil moisture lead to lower yields at higher temperatures. Intensified soil and water resource degradation resulting from changes in temperature and precipitation will further stress agriculture in certain regions.25
- Adapted from image by W. Elder, National Park Service.
- U.S. Global Change Research Program (USGCRP) (2009) Global Climate Change Impacts in the U.S.
- National Oceanic and Atmospheric Administration (NOAA) (2019) “What’s the Difference Between Weather and Climate?”
- National Aeronautics and Space Administration (2010) The Earth’s Radiation Budget.
- Intergovernmental Panel on Climate Change (IPCC) (2013) Climate Change 2013: The Physical Science Basis.
- CSS calculation based on data from UN Environment Programme (UNEP) and UN Framework Convention on Climate Change (UNFCCC) (2003) Climate Change Information Kit.
- U.S. Environmental Protection Agency (EPA) (2016) Climate Change Indicators in the U.S., 2016.
- UNEP (2012) Policy Implications of Warming Permafrost.
- Cappucci, M. (2020) “Unprecedented heat in Siberia pushed planet to warmest June on record, tied with last year.” The Washington Post.
- IPCC (2014) Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.
- Hansen, J., et al. (2005) Earth’s Energy Imbalance: Confirmation and Implications. Science, 229(3): 857.
- Archer, D., et al. (2009) Atmospheric Lifetime of Fossil Fuel Carbon Dioxide. Annual Review of Earth and Planetary Sciences, 37: 117-34.
- UNEP and GRID-Arendal (2005) Vital Climate Change Graphics.
- Adapted from USGCRP (2009) Global Climate Change Impacts in the United States.
- NOAA (2021) State of the Climate: 2020 Global Climate Report.
- NOAA (2020) Arctic Report Card 2020.
- USGCRP (2018) Fourth National Climate Assessment.
- Photo courtesy of the National Snow and Ice Data Center/World Data Center for Glaciology.
- Secretariat of the Convention on Biological Diversity (2010) Global Biodiversity Outlook 3.
- National Research Council (2009) Ecological Impacts of Climate Change.
- U.S. EPA (2021) Climate Change Indicators: Length of Growing Season.
- IPCC (2018) Global Warming of 1.5 C: Summary for Policy Makers, Chapter 1.
- Cao, L., et al. (2014) Response of ocean acidification to a gradual increase and decrease of atmospheric CO2. Environmental Research Letters, 9(2), 1-9.
- IPCC (2007) Climate Change 2007: Impacts, Adaptation and Vulnerability. Working Group II Contributions to the IPCC Fourth Assessment Report.
- National Research Council (2011) Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia.