9.2 Causes and Evidence of Climate Change

Climate change is reshaping the planet. Human activities, especially burning fossil fuels and deforestation, are increasing greenhouse gas concentrations in the atmosphere far faster than any natural process. Understanding what's driving this change and how we know it's happening is central to environmental science.

Natural climate cycles do exist, but the current rate of warming far outpaces anything in the paleoclimate record. Evidence from ice cores, tree rings, ocean chemistry, and rising sea levels all converge on the same conclusion: the climate is changing rapidly, and human activity is the primary driver.

Human Causes of Climate Change

Anthropogenic Greenhouse Gas Emissions

Anthropogenic means "caused by humans." Anthropogenic climate change refers to the warming driven by human activities that increase greenhouse gas concentrations in the atmosphere. Two gases matter most here: carbon dioxide and methane.

Carbon dioxide ($CO_2$) is the biggest contributor to human-caused warming:

• Burning fossil fuels (coal, oil, natural gas) for energy releases $CO_2$ that was locked underground for millions of years
• Industrial processes like cement production generate large amounts of $CO_2$ as a chemical byproduct
• The transportation sector (cars, trucks, planes) accounts for about 14% of global greenhouse gas emissions

Methane ($CH_4$) traps roughly 80 times more heat than $CO_2$ over a 20-year period, making it extremely potent even in smaller quantities:

• Livestock (especially cattle) produce methane through enteric fermentation, a digestive process in their stomachs
• Landfills release methane as organic waste decomposes without oxygen
• Natural gas production and distribution systems leak methane at multiple points
• Flooded rice paddies create oxygen-free (anaerobic) conditions where methane-producing microbes thrive

Land Use Changes and Deforestation

Forests act as carbon sinks, meaning they absorb more $CO_2$ than they release. When forests are cleared, that process works in reverse.

• Tropical rainforests like the Amazon and Congo Basin store enormous amounts of carbon in their biomass. Clearing them for agriculture or development releases that stored carbon back into the atmosphere.
• Deforestation also reduces the planet's ongoing capacity to pull $CO_2$ out of the air, and it disrupts local climate patterns and biodiversity.

Urban expansion adds another layer. Concrete and asphalt absorb and retain more heat than natural surfaces like soil and vegetation. This creates the urban heat island effect, where cities run noticeably warmer than surrounding rural areas.

Natural Climate Variability

Long-term Climate Cycles

Earth's climate has always fluctuated, but these natural cycles operate on very different timescales than what we're seeing now.

• Milankovitch cycles are slow changes in Earth's orbit and axial tilt that alter how much solar radiation different parts of the planet receive. These cycles range from about 20,000 to 100,000 years and have driven the ice ages.
• El Niño Southern Oscillation (ENSO) causes shorter-term fluctuations every 2–7 years by shifting ocean temperatures in the tropical Pacific. ENSO events alter global weather patterns, affecting temperature and precipitation across many regions.
• Solar activity varies on an approximately 11-year sunspot cycle, slightly changing Earth's energy balance. Long-term shifts in solar output can influence climate trends, but these changes are small compared to the warming caused by greenhouse gases.

The key distinction: natural cycles cause gradual shifts over thousands of years. Current warming has occurred over just a century or two, which is essentially instantaneous in geological terms.

Paleoclimate Records and Proxies

Scientists can't directly measure temperatures from thousands of years ago, so they use proxy records, natural archives that preserve clues about past climate conditions.

• Ice cores from Greenland and Antarctica contain tiny trapped air bubbles that reveal the actual composition of the atmosphere at the time the ice formed. Oxygen isotope ratios in the ice indicate past temperatures. Some cores span over 800,000 years of climate history.
• Tree rings record annual growth patterns. Wider, denser rings generally indicate favorable temperature and moisture conditions; narrow rings suggest stress from drought or cold. Some tree ring records extend back thousands of years.
• Sediment cores from lake beds and ocean floors contain microfossils and chemical signatures that indicate past environmental conditions. These records can span millions of years.

Together, these proxies show that current $CO_2$ levels (over 420 ppm) are higher than at any point in at least the last 800,000 years.

Evidence and Impacts of Climate Change

Observed Changes in Earth Systems

Multiple independent lines of evidence confirm that the climate is warming.

• Sea level rise: Global average sea level has risen about 8–9 inches since 1880. This is caused by two things: thermal expansion (warmer water takes up more volume) and melting land ice from glaciers and ice sheets. Projections suggest sea levels could rise 1–4 feet by 2100, threatening coastal communities and island nations.
• Global temperature increase: Earth's average surface temperature has risen by about 1°C (1.8°F) since pre-industrial times. The rate of warming has accelerated in recent decades, with 2016 and 2020 tied as the warmest years on record.
• Arctic sea ice decline: September minimum sea ice extent has decreased by about 13% per decade since satellite measurements began in 1979. This loss affects global climate patterns and the ecosystems that depend on ice cover.

Ocean and Ecosystem Impacts

• Ocean acidification occurs because the oceans absorb roughly 30% of atmospheric $CO_2$. When $CO_2$ dissolves in seawater, it forms carbonic acid, lowering the ocean's pH. This makes it harder for organisms like corals, oysters, and many plankton species to build their calcium carbonate shells and skeletons. Since these organisms form the base of many marine food webs, the effects ripple upward.
• Species are shifting their ranges. Many plants and animals are moving toward the poles or to higher elevations to track suitable temperatures. Seasonal timing is also changing: migration, flowering, and breeding events are occurring earlier in many species. This can create dangerous mismatches, for example, if insects emerge before the birds that feed on them arrive.
• Extreme weather events are becoming more frequent and intense in many regions. This includes more severe heatwaves and droughts, stronger tropical cyclones in some ocean basins, and altered precipitation patterns that increase the risk of both floods and landslides.