Climate Change: Science, Observed Impacts, and Future Projections Study Guide

The Science and Mechanism of the Earth’s Climate

• Climate Change vs. Weather Definitions:

  • Weather: This refers to atmospheric conditions observed at a specific location on a very short time scale, typically hours or days. It is characterized by a high degree of variability.

  • Climate: This is the long-term average of short-term weather patterns. Measures include annual average temperature or rainfall over extended periods.

• The Global Energy Balance:

  • Under stable conditions, the Earth maintains an energy balance between incoming short-wave solar radiation and outgoing long-wave infrared radiation.

  • Solar radiation passes through the atmosphere; the majority is absorbed by the Earth's surface.

  • The surface re-emits this energy as infrared radiation. While a portion escapes into space, greenhouse gases (GHGs) in the atmosphere trap some of this radiation.

  • Mechanism of Warming: Increasing the concentration of GHGs reduces the amount of energy radiated back to space, thus warming the planet.

• The Unprecedented Nature of Current Change:

  • Climate change is currently altering precipitation, temperature, and sea levels.

  • There has been an unprecedented acceleration of change over the last $50$ years.

  • Global climate models show increasing confidence that human-led GHG emissions are the primary driver.

Climate Forcings and Human Influence

• Definition of Forcings: Disturbances to the Earth’s energy balance are known as climate forcings.

  • Positive Forcings: Exert a warming influence. Greenhouse gases are the primary example.

  • Negative Forcings: Exert a cooling influence. Sulfate aerosols are a primary example.

• Greenhouse Gas Potency and Impact:

  • Carbon Dioxide ($CO_2$): While GHGs like methane are more potent on a per-molecule basis, $CO_2$ contributes most to global warming due to its high prevalence in the atmosphere.

  • Anthropogenic Forcing Magnitude: Human-related GHG emissions represent a climate forcing roughly equal to $1\%$ of net incoming solar energy. This is energetically equivalent to burning $13\text{ million barrels of oil every minute}$.

• Attribution of Warming:

  • Natural forcings alone (such as solar variation and volcanic activity) cannot explain the warming observed since $1850$.

  • Anthropogenic forcings (GHGs and aerosols) also cannot fully explain the trend in isolation.

  • Climate models only match observed temperature trends when both natural and anthropogenic forcings are considered together.

• IPCC 2023 Conclusion: The Intergovernmental Panel on Climate Change (IPCC) stated that human activities have "unequivocally" caused global warming. Global surface temperatures reached $1.1^\circ\text{C}$ above the $1850$-$1900$ baseline during the period of $2011$-$2020$.

Climate Feedbacks, Inertia, and Momentum

• Climate Feedbacks: These are the Earth's responses to initial forcings that can amplify or dampen warming.

  • Water Vapor: As the planet warms, evaporation increases, leading to more water vapor in the atmosphere. Since water vapor is a powerful GHG, this further increases surface warming.

  • Albedo Effect: As polar ice melts, the Earth's surface becomes less reflective. The oceans then absorb more solar radiation, accelerating warming.

  • Permafrost Decay: Increased temperatures cause organic matter in subarctic permafrost to thaw and decay, releasing additional $CO_2$ and methane.

  • Siberia Case Study (June 2020): This month tied for the warmest on record. Extreme temperatures in Siberia led to large wildfires and permafrost thawing. The fires alone released an estimated $59\text{ million metric tons (Mt)}$ of $CO_2$ into the atmosphere.

• System Inertia and Persistence:

  • Ocean Thermal Inertia: The massive volume of the world's oceans slows the climate's response to forcings, creating a delayed response with high momentum.

  • Response Time: It takes approximately $25$ to $50\text{ years}$ for the Earth's temperature to reach $60\%$ of its equilibrium response to a forcing.

  • Atmospheric Persistence: $CO_2$ persists in the atmosphere for hundreds of years. Consequently, even if emissions were eliminated today, impacts would continue for centuries.

Observed Impacts on Physical Systems

• Temperature Trends:

  • In $2022$, global average temperatures were $0.86^\circ\text{C}$ ($1.55^\circ\text{F}$) higher than the $20\text{th}$ century average.

  • $2016$ stands as the warmest year on record since $1880$, with $2020$ ranking second.

  • The nine warmest years on record have all occurred between $2014$ and $2022$.

  • $2022$ marked the $46\text{th}$ consecutive year with above-average annual global temperatures.

• Cryosphere and Arctic Changes:

  • Arctic temperatures in $2022$ rose to $0.73^\circ\text{C}$ above the $1991$-$2020$ average.

  • Arctic sea ice is thinner, younger, and less expansive than in the $1980\text{s}$ and $90\text{s}$.

  • The $2021$ ice extent was the $12\text{th}$ lowest on record since $1979$, covering only $4.92\text{ million square kilometers}$.

  • Snow cover in the Northern Hemisphere has decreased by $1\%$ relative to $1850$-$1900$. Under $4^\circ\text{C}$ of warming, it is predicted to decrease by $15\%$ to $30\%$.

• Hydrology and Sea Level:

  • US Precipitation: Average annual precipitation has increased by $4\%$ since $1901$, with a significant increase in the intensity and frequency of extreme events.

  • Sea Level Rise: Global mean sea level rose between $15$ and $25\text{ cm}$ since $1901$. Due to ice sheet melt and deep ocean warming, further rise is unavoidable for centuries or millennia.

Observed Impacts on Biological Systems

• Phenology and Range: Warming is altering biological timing (phenology) and the geographic distribution of species.

• Geographic Shifts: Approximately half of the species assessed globally have shifted their ranges polewards or toward higher elevations.

• Ecological Relationships: Predator-prey interactions are being disrupted, particularly when climate-driven shifts occur at different rates or times for different species.

• Growing Season: In the contiguous $48$ states of the U.S., the average growing season has lengthened by nearly $2\text{ weeks}$ since the start of the $20\text{th}$ century.

Future Projections and Predicted Changes

• Temperature Projections:

  • The IPCC predicts a global temperature rise of $1.5^\circ\text{C}$ ($2.7^\circ\text{F}$) by the early $2030\text{s}$.

  • Long-term projections (relative to $1986$-$2005$):

    • $2045$-$2065$: Increase of $0.4$-$-$2.6^\circ\text{C}$($0.7$-$-$4.7^\circ\text{F}$).

    • $2081$-$2100$: Increase of $0.3$-$-$4.8^\circ\text{C}$($0.5$-$-$8.6^\circ\text{F}$).

  • The modern rate of warming ($1.7^\circ\text{C per century}$ since $1970$) is vastly higher than the historical rate of decline ($0.01^\circ\text{C}$ over the past $7,000\text{ years}$).

• Ocean Projections:

  • By $2100$, sea level is modeled to rise between $26$ and $77\text{ cm}$ for a $1.5^\circ\text{C}$ temperature increase.

  • Acidification: Oceans absorb about $31\%$ of anthropogenic $CO_2$ emissions, increasing acidity.

  • Coral Reefs: Projected to decline by $70$-$-$90\%$under a$1.5^\circ\text{C} warming scenario.\n\n# Implications for Human and Natural Systems\n\n* **Ecosystem Resilience:** The combination of climate change and other global drivers will likely exceed the resilience capacity of many ecosystems this century.\n* **Extreme Events:** A 4^\circ\text{C} warming scenario involves more frequent/intense hot and cold temperature extremes, droughts, hurricanes, and precipitation events.\n* **Temperature Extremes:** With a 2^\circ\text{C}$increase, nearly every summer would be warmer than the hottest$5\% of summers in the recent past.\n* **Human Health:**\n * Increased occurrences of food-borne and water-borne diseases.\n * Vector-borne diseases are appearing more frequently and in new geographic regions.\n* **Agriculture and Food Security:**\n * Initial boosts in crop yields from higher CO_2 and slight warming are offset by negative effects of high heat and reduced soil moisture.\n * Increased temperatures lead to lower yields overall and intensified degradation of soil and water resources.\n* **Sustainability Warning:** The IPCC stated in 2023$$ with very high confidence: "There is a rapidly closing window of opportunity to secure a livable and sustainable future for all."