Climate change

The Global Carbon Cycle: Mechanisms and Reservoirs

• Definition: The global carbon cycle describes the movement of carbon between three primary locations: terrestrial ecosystems, the oceans, and the atmosphere.

• Biological Processes of Carbon Exchange:
    - Photosynthesis: The process that incorporates carbon from the atmosphere into living tissue. This incorporates carbon into organic forms.
    - Cellular Respiration: The process that releases carbon from living organisms back into the atmosphere in the form of carbon dioxide ($CO_2$).

• Primary Carbon Reservoirs (measured in gigatons or $10^{15}$ grams of carbon):
    - Sedimentary Rocks: The largest reservoir, containing approximately $100,000,000$ gigatons.
    - The Oceans: A major reservoir containing approximately $37,000$ gigatons of carbon (includes organisms and chemical processes).
    - Terrestrial Organisms, Soil, Litter, and Peat: Contains approximately $3300$ gigatons.
    - Sediments: Contains approximately $1750$ gigatons.
    - Fossil Fuels: Contains approximately $1000$ gigatons.
    - Atmosphere: A small but critical reservoir containing approximately $870$ gigatons of $CO_2$ (representing roughly $0.04 ext{ extperthousand}$ of the atmosphere).

• Estimated Yearly Carbon Fluxes (gigatons of carbon per year):
    - Photosynthesis (Terrestrial): Net uptake of $5.3$ gigatons/year.
    - Ocean Uptake: Net uptake through photosynthesis and chemical processes of $1.9$ gigatons/year.
    - Respiration and Fire: Natural release from terrestrial systems.
    - Weathering and Sedimentation: Movement of carbon from land to sediments and vice versa.

Human Impacts on the Global Carbon Cycle

• Primary Anthropogenic Alterations:
    - Humans are profoundly changing the global carbon cycle through three main activities:
      1. Fossil-fuel use: Contributing approximately $9.4$ gigatons/year to the atmosphere.
      2. Land-use change: Primarily deforestation, contributing approximately $1.6$ gigatons/year.
      3. Fire: Anthropogenic burning contributes to increased atmospheric carbon.

• Historical Trends in Atmospheric $CO_2$:
    - Pre-industrial Levels: Around the year $1400 ext{ to }1800$, carbon dioxide concentrations were steady at approximately $280$ ppm.
    - Post-industrial Increase: Since the mid-$1800s$, there has been a sharp increase in carbon emissions from fossil fuel burning, rising from near-zero to over $10,000$ million metric tons annually by $2026$.
    - Current Concentration: Atmospheric $CO_2$ concentration has surpassed $420$ ppm as of approximately $2026$. This trend is clearly visible in graphs covering both the past $200$ years and the past decade.

Understanding Global Climate Change and the Greenhouse Effect

• Terminology and Definitions:
    - Global Warming: Refers specifically to the increase in the average temperature of the entire planet.
    - Global Climate Change: Refers to the sum of all local changes in temperature and precipitation patterns that result as a consequence of global warming.
    - Weather vs. Climate: These are distinct concepts. Climate refers to the long-term average patterns, whereas weather refers to short-term, local conditions.

• The Greenhouse Effect Mechanism:
    - Step 1: High-energy solar radiation enters the Earth's atmosphere.
    - Step 2: Some radiation is reflected, while the rest is absorbed by the Earth's surface, warming it.
    - Step 3: The Earth emits heat as low-energy infrared radiation.
    - Step 4: Some of this heat escapes into space, but much of it is trapped by greenhouse gases (such as $CO_2$) and retained in the atmosphere, increasing the global temperature.

• Scientific Concerns:
    - While average atmospheric temperatures and weather patterns have fluctuated throughout Earth's history, current concerns stem from two factors:
      1. The rate of change is unprecedented.
      2. The change is clearly caused by human activities.
    - The current warming trend cannot be explained by fluctuations in solar energy alone.

Physical and Climatic Effects of Global Warming

• Documented and Predicted Warming:
    - A warming of $1.25^ ext{C}$ has already been documented compared to historical baselines.
    - The Pleistocene epoch shows historical dips representing ice ages, but the current trajectory shows temperatures rising far above historic norms.
    - Future projections depend on emissions: dramatic reduction of greenhouse gas emissions could stabilize temperatures, while continued very high emissions would lead to an increase of $8^ ext{C}$ or more.

• Primary Climatic Effects:
    1. Warming: Average temperatures will continue to rise globally, though not uniformly; some regions will experience more intense warming than others.
    2. Temperature Fluctuations: In addition to rising averages, temperature extremes (both heat waves and localized anomalies) are expected to increase.
    3. Regional Drought: Changes in precipitation and soil moisture patterns are expected. Predictions show:
       - At $2^ ext{C}$ warming: Significant areas experience drier soil moisture.
       - At $4^ ext{C}$ warming: Drastic increases in the severity and geographic extent of drought (noted as "much drier").

• Feedback Loops: Positive feedback loops (where the effects of warming lead to further warming) are a significant concern in climate models.

Ecological and Biological Consequences

• Ecological Effects Categories:
    1. Geographic Range Shifts: Species are moving to new locations as their original habitats become unsuitable. An example includes the expansion area of Lyme disease, linked to the movement of tick vectors.
    2. Phenology Shifts: The timing of seasonal events is changing. Phenology refers to biological rhythms such as flowering times, migration, or egg-laying. Disruptions occur when dependent species (e.g., a bird and its insect food source) shift their timing at different rates.
    3. Evolutionary Adaptation: Changes in allele frequencies within populations are being observed.
       - Case Study: The frequency of the "black morph" (melanic form) in a lady beetle population decreased from $60 ext{ extpercent}$ to $20 ext{ extpercent}$ in just $30$ years as an evolutionary response to a changing environment.
    4. Extinction: Populations that cannot migrate or adapt fast enough face local or global extinction.
    5. Ocean Acidification: Increased atmospheric $CO_2$ is absorbed by the oceans, lowering the pH and impacting marine life, particularly those with calcium carbonate shells.

Global Emissions Data and Regional Responsibility

• The Role of the United States: The US has an outsized role in mitigating global climate change based on several metrics:
    - Population size: While the US population is smaller than China and India ($1000 ext{--}1500$ millions), it is significantly larger than many other developed nations.
    - Per Capita $CO_2$ Emissions: The United States has one of the highest per capita rates, exceeding $15$ metric tons per person, which is higher than China, India, and European nations like Germany, France, and Japan.
    - Total $CO_2$ Emissions: Due to high per capita emissions, the US remains a top global emitter in terms of total million metric tons of carbon, alongside China.