Global climate change is a pressing issue driven by human activities, primarily the emission of greenhouse gases. This topic explores the causes of climate change, including anthropogenic sources of CO2, methane, and other greenhouse gases that trap heat in the atmosphere.

Evidence of climate change is seen in rising temperatures, sea levels, and changes in weather patterns. We'll examine temperature and sea level observations, as well as paleoclimate proxies like ice cores and tree rings that reveal Earth's climate history.

Greenhouse Gases and Emissions

Anthropogenic Greenhouse Gas Sources

• Greenhouse gases trap heat in the atmosphere by absorbing and emitting infrared radiation
• Carbon dioxide emissions have increased dramatically since the industrial revolution due to burning fossil fuels (coal, oil, natural gas)
• Methane is a potent greenhouse gas released from agricultural activities (livestock, rice cultivation), landfills, and natural gas production
  • Methane has a shorter atmospheric lifetime than CO2 but a higher global warming potential per molecule
• Deforestation reduces the Earth's capacity to absorb CO2 through photosynthesis and releases the carbon stored in trees back into the atmosphere
• Industrial revolution marked the beginning of large-scale fossil fuel use and a rapid increase in atmospheric CO2 concentrations
  • Pre-industrial CO2 levels were around 280 ppm, now over 400 ppm
• Radiative forcing measures the change in Earth's energy balance due to factors like greenhouse gases
  • Positive radiative forcing from anthropogenic emissions is driving global warming

Other Greenhouse Gases and Their Sources

• Nitrous oxide is released from agricultural soil management (fertilizers), industrial processes, and combustion
• Ozone in the lower atmosphere acts as a greenhouse gas and is formed by chemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight
  • Major sources include vehicle exhaust and industrial emissions
• Water vapor is the most abundant greenhouse gas but its atmospheric concentration is not directly affected by human activities
  • However, as the Earth warms, more water evaporates leading to a positive feedback loop
• Chlorofluorocarbons (CFCs) are powerful greenhouse gases that also deplete the ozone layer
  • Now regulated by the Montreal Protocol, but some of their replacements (HFCs) are also strong greenhouse gases

Evidence of Climate Change

Temperature and Sea Level Observations

• Global warming refers to the long-term trend of rising average global temperatures due to the greenhouse effect
  • Earth's average surface temperature has increased by about 1°C since the pre-industrial era
• Sea level rise is caused by thermal expansion of ocean water as it warms and the melting of land-based ice (glaciers, ice sheets)
  • Global mean sea level has risen about 21-24 cm since 1880
• Temperature anomalies show how much warmer or cooler a region is compared to a long-term average
  • The past five years (2016-2020) have been the warmest on record globally

Paleoclimate Proxies

• Ice core samples provide a record of past atmospheric composition and temperature
  • Air bubbles trapped in ice cores allow direct measurement of past CO2 levels
  • The ratio of oxygen isotopes in the ice can be used to infer past temperatures
• Tree rings are annual growth rings that are wider during warm, wet years and narrower during cold, dry years
  • The width and density of tree rings can be used to reconstruct past climate conditions
  • Older trees (like bristlecone pines) can provide climate records going back thousands of years

Climate History and Feedbacks

Earth's Climate History

• Paleoclimate refers to past climate conditions and the study of how Earth's climate has changed over geologic time scales
• Earth has experienced natural climate variations due to factors like changes in solar output, volcanic eruptions, and shifts in Earth's orbit (Milankovitch cycles)
  • Ice ages are an example of natural climate change, characterized by colder global temperatures and the growth of continental ice sheets
• However, the current rate and magnitude of warming is unprecedented in at least the last 2,000 years and cannot be explained by natural factors alone
  • The scientific consensus attributes the observed warming to human activities, primarily greenhouse gas emissions

Climate Feedbacks

• Feedback loops in the climate system can amplify (positive feedback) or dampen (negative feedback) the initial change
• Examples of positive feedbacks that accelerate warming:
  • Ice-albedo feedback: As Arctic sea ice melts, it exposes darker ocean water that absorbs more sunlight, causing further warming and ice loss
  • Water vapor feedback: Warmer air can hold more water vapor (a greenhouse gas), which traps more heat and causes additional warming
• An example of a negative feedback that slows warming:
  • Planck feedback: As Earth's surface warms, it emits more infrared radiation to space, helping to cool the planet
• The net effect of all feedbacks will determine the ultimate magnitude of climate change for a given increase in greenhouse gases