Climate Change: Feedbacks, Processes, and History

Climate Feedbacks and Their Impact on Climate Change

Feedbacks within the climate system amplify climate changes initially caused by external factors. This can be visualized as:

• Initial climate forcing → Initial climate response → Response amplified

Example: An increase in heat energy sent to Earth by the Sun causes high-latitude snow and ice to retreat. This allows more sunlight to be absorbed by the Earth's surface, leading to further climatic warming (amplifying the changes underway). Another example is delayed bedrock rebound.

Negative Feedback Mechanisms in Climate Change

Initial climate changes can also lead to a reduced response:

• Initial climate forcing → Initial climate response → Response reduced

Example: Chemical weathering acts as a negative feedback mechanism by reducing the intensity of imposed climate warming and cooling.

Chronological Dating Methods in Paleoclimatology

Various methods are used to establish chronologies in climate studies:

• Radiometric dating
• Radiocarbon dating
• Counting annual layers
• Correlating records with orbital cycles
• Internal chronometers
• Sediment archives
• Ice cores
• Tree rings
• Corals
• Biotic data (e.g., macrofossils, pollen, plankton, geological and geochemical data)

Using Plant Leaves to Determine Past Climates

• Warmer climates: Palm-like leaves
• Colder climates: Serrated, jagged leaves
• Warm climates: Round, smooth leaves

Isotopes as Climate Proxies

Isotopes, particularly in ocean shells (foraminifera/coral), provide valuable climate data:

• Oxygen isotopes: Indicate temperatures and the amount of ice.
• Carbon isotopes: Track the transfer/change of organic material to land and sea.

The Faint Young Sun Paradox

The Faint Young Sun Paradox questions how liquid water existed on early Earth despite a weaker sun. The paradox suggests that carbon reservoirs change (residing in different reservoirs) and greenhouse gases contribute like a thermostat.

A stronger greenhouse effect, potentially due to higher atmospheric CO2 levels, could have compensated for the weaker solar radiation.

Hydrolysis and the Carbon Cycle

Hydrolysis is a chemical weathering process where H+ and -OH ions from water break down and substitute other ions in silicate rocks. Silicon (Si) and Calcium (Ca) remain, combining with CO2 to form calcium carbonate. Diatoms are an example of organisms involved in this process.

Three key ingredients in hydrolysis:

1. Materials that make up continental rock
2. Water derived from rain
3. CO2 derived from the atmosphere

Hydrolysis plays a crucial role in the long-term carbon cycle by pulling CO2 out of the atmosphere and storing it in sediments and rocks. Acid rain, formed by rain and atmospheric CO2, facilitates this process. CO2 breaks down silicates, which are washed into oceans and used by diatoms, ultimately recreating rocks and storing carbon.

The Weathering Thermostat

The weathering thermostat moderates long-term climate change. It is controlled by temperature, precipitation, and vegetation. This thermostat also functions as a negative feedback mechanism.

• The Gaia hypothesis proposes that life regulates climate.
• A malfunction of the thermostat could lead to a "snowball Earth" scenario.
• The weathering thermostat is a crucial element in resolving the Faint Young Sun Paradox.

Negative Feedback Loop:

1. Increased atmospheric CO2 (e.g., from volcanic activity) leads to an increase in temperature, precipitation, and vegetation.
2. Increased weathering removes CO2 from the atmosphere.
3. Less CO2 in the atmosphere causes a decrease in temperature, precipitation, and vegetation.
4. When less CO2 is removed from the atmosphere, the atmosphere begins warming up again.

The BLAG Hypothesis

The BLAG hypothesis (named after Berner, Lasaga, and Garrels) suggests that climate change over the last several hundred million years has been driven by CO2 input into the atmosphere and ocean through plate tectonic processes:

• CO2 removal by chemical weathering.
• CO2 returned to the atmosphere by volcanic activity.
• Seafloor spreading as a driver: Faster spreading leads to more CO2 release.

The carbon cycle provides long-term stability to the climate, balancing burial and chemical weathering.

Uplift and Weathering

Uplift influences chemical weathering, acting not just as a negative feedback to BLAG, but also as a driver of climate change. The rate of chemical weathering is heavily affected by the availability of fresh surface rocks and minerals. Mountainous regions with steep slopes experience mass wasting (and precipitation).

Faster uplift equates to greater weathering and a cooler climate.