7.2 Evidence and impacts of global warming

Global warming is reshaping our planet. Evidence from temperature records, ice cores, and satellite data shows a clear warming trend. The impacts are far-reaching, affecting ecosystems, sea levels, and human societies worldwide.

Climate change is pushing Earth towards tipping points. Melting ice sheets, Amazon dieback, and ocean circulation changes could trigger irreversible shifts. These cascading effects underscore the urgency of addressing global warming and its consequences.

Evidence for Global Warming

Temperature and Atmospheric Records

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• Global temperature records reveal clear warming trend over past century
  • Last few decades warmest on record
  • Unprecedented rate of warming compared to historical climate variability
• Ice core samples provide historical record of atmospheric CO2 concentrations
  • Sharp increase coincides with industrialization (late 18th century)
  • Current CO2 levels exceed 410 ppm, highest in at least 800,000 years
• Isotopic analysis of atmospheric CO2 confirms anthropogenic origin
  • Distinct isotopic signature of fossil fuel-derived carbon
  • Demonstrates human activities as primary driver of increased greenhouse gas concentrations

Cryosphere and Ocean Changes

• Satellite measurements demonstrate decrease in Arctic sea ice
  • Extent reduced by approximately 40% since 1979
  • Thickness decreased by 65% since 1975
• Mass loss from ice sheets in Greenland and Antarctica
  • Greenland losing about 280 billion tons of ice per year
  • Antarctica losing about 150 billion tons of ice per year
• Ocean heat content increased significantly
  • Absorbs more than 90% of excess heat trapped by greenhouse gases
  • Warming extends to depths of 2000 meters and beyond

Biological and Paleoclimate Evidence

• Phenological changes provide biological evidence of warming trends
  • Earlier spring blooming (cherry blossoms in Japan)
  • Later fall leaf drop (New England deciduous forests)
  • Shifts in animal migration patterns (earlier arrival of migratory birds)
• Paleoclimate data corroborate current warming rates as unprecedented
  • Tree rings reveal temperature patterns over centuries
  • Sediment cores provide insights into ocean temperatures and circulation
  • Coral reefs record changes in sea surface temperatures and ocean chemistry

Climate Change Impacts

Ecosystem and Biodiversity Effects

• Shifts in species ranges and phenology disrupt ecological relationships
  • Poleward and upslope migration of plant and animal species
  • Potential mismatches between interdependent species (pollinators and flowering plants)
• Coral reef ecosystems face increased bleaching events and potential collapse
  • Warming causes expulsion of symbiotic algae, leading to bleaching
  • Ocean acidification reduces ability of corals to build calcium carbonate skeletons
• Thawing permafrost releases stored carbon and methane
  • Creates positive feedback loop accelerating warming
  • Alters landscape and damages infrastructure in Arctic regions

Human Society Impacts

• Sea level rise threatens coastal ecosystems and human populations
  • Increased vulnerability to flooding and storm surges (Venice, Miami)
  • Saltwater intrusion into freshwater aquifers (Bangladesh, Maldives)
• Changes in precipitation patterns affect agricultural productivity
  • Altered growing seasons and crop yields (Midwest United States, Sub-Saharan Africa)
  • Increased frequency of droughts and floods impacts food security
• Vector-borne diseases expand their ranges
  • Malaria and dengue fever moving into higher altitudes and latitudes
  • Emergence of new zoonotic diseases due to shifting ecosystems
• Climate-induced migration and displacement of human populations
  • Environmental refugees from low-lying island nations (Tuvalu, Kiribati)
  • Increased pressure on urban infrastructure and resources in receiving areas

Climate Tipping Points

Cryosphere Tipping Points

• Melting of Greenland ice sheet approaching tipping point
  • Could lead to several meters of sea level rise over centuries to millennia
  • Positive feedback loop as lower elevation ice melts faster
• Collapse of West Antarctic Ice Sheet
  • Potential for rapid sea level rise of up to 3.3 meters
  • Grounding line retreat and ice shelf disintegration accelerate ice loss

Biosphere Tipping Points

• Dieback of Amazon rainforest due to drought and deforestation
  • Release of massive amounts of stored carbon
  • Alteration of global precipitation patterns
  • Loss of biodiversity hotspot and ecosystem services
• Large-scale permafrost thaw
  • Release of vast quantities of methane, a potent greenhouse gas
  • Creation of amplifying feedback loop accelerating warming
  • Landscape changes and infrastructure damage in Arctic regions

Ocean and Atmospheric Circulation Tipping Points

• Shutdown or significant weakening of Atlantic Meridional Overturning Circulation (AMOC)
  • Dramatic alteration of climate patterns in Europe and beyond
  • Potential cooling of North Atlantic region despite global warming
  • Disruption of marine ecosystems and fisheries
• Changes in El Niño-Southern Oscillation (ENSO) patterns
  • Intensification of extreme weather events (droughts, floods)
  • Alterations in global temperature and precipitation distributions

Observed and Projected Climate Changes

Temperature and Sea Level Changes

• Global average temperatures increased by approximately 1°C since pre-industrial times
  • Projections indicate potential warming of 1.5°C to 4°C by 2100
  • Depends on emission scenarios and climate sensitivity
• Sea levels risen by about 20 cm since 1900
  • Current rates of rise accelerating to 3.6 mm per year
  • Projections suggest further rise of 0.3 to 2.5 meters by 2100
  • Depends on emissions and ice sheet stability

Weather and Atmospheric Pattern Shifts

• Extreme weather events increased in frequency and intensity
  • More frequent and severe heatwaves (European heatwave of 2003)
  • Intensification of droughts (California drought 2011-2017)
  • Increase in heavy precipitation events (Hurricane Harvey 2017)
• Changes in atmospheric circulation patterns
  • Shifts in storm tracks alter regional precipitation patterns
  • Weakening of jet stream leads to more persistent weather patterns
• Arctic amplification results in polar warming rates twice the global average
  • Leads to permafrost thaw and changes in polar ecosystems
  • Alters global temperature gradients and atmospheric circulation

Ocean and Cryosphere Changes

• Ocean acidification increased by about 30% since industrial revolution
  • Further decreases in pH projected as CO2 concentrations rise
  • Impacts on marine calcifying organisms (corals, shellfish)
• Alterations in ocean currents, such as Gulf Stream
  • Potential weakening of Atlantic Meridional Overturning Circulation
  • Impacts on regional climate patterns and marine ecosystems
• Rapid decline in Arctic sea ice extent and volume
  • Potential for ice-free Arctic summers by mid-century
  • Albedo feedback accelerates regional warming