Anthropogenic Climate Change: Notes

In the previous lecture, natural sources of global warming and the importance of natural variability in modern climate change were discussed.
This lecture will focus on anthropogenic causes of global warming.
- The physics of greenhouse gases (GHGs) have been understood since the 1800s.
- Increased GHG concentrations lead to more infrared radiation (IR) being trapped, resulting in higher temperatures.
- Feedbacks play a crucial role, as previously discussed.
Since 1750, CO2 levels have increased by 50%, from 280 to 425 ppm.
- This increase needs to be compared with CO2 levels over the past 800,000 years.
- This increase will inevitably affect global temperatures.
- Other forcings and feedbacks also contribute to climate change.
- The increase in CO2 acts as both a forcing and a feedback mechanism.

The top three panels show three natural forcings over the past 100 years.
- What is their relative importance?
- Each exhibits short-term variability.
- Which have a long-term trend?
The bottom panel shows human forcings.
- Short-term vs. long-term variability needs to be considered.
- How important are human forcings compared to natural ones?
These forcings provide insight into the global temperature record.

The figure displays monthly global temperature anomalies since the 1940s.
- What are the main points?
- El Niño occurred from July 2023 to April 2024. What was its impact?
Many global temperature records were set in the past two years.
- 2023: Hottest summer and hottest year on record.
- 2024: Hottest day (July 23rd), hottest summer, and hottest year.

Climate scientists use complex computer models, called General Circulation Models (GCMs), to simulate the climate system and forcings.
These models are not perfect but are increasingly sophisticated and can reproduce 20th-century climate.
Different models are run multiple times to generate a range of possible outcomes.
GCMs are used for temperature change attribution, examining annual mean temperature anomalies and annual ocean heat content.
Measurements are compared with GCM results for two scenarios:
- Natural forcings only (solar, volcanic, etc.).
- Natural and anthropogenic forcings.
The data representation:
- Measurements (including proxies) are shown as a black line.
- GCM results with only natural forcings are shown as a green line.
- GCM results with natural and anthropogenic forcings are shown as a brown line.

How well do GCMs with only natural forcings match observations?
How well do GCMs with both natural and anthropogenic forcings match observations?
Conclusion (e.g., from the IPCC AR6 report):
- Air Temperatures: Comparison of GCM results with observations.
- Global Ocean Heat Content: Comparison of GCM results with observations.
- Air Temperatures (Arctic): Comparison of GCM results with observations.
- Air Temperatures (North America): Comparison of GCM results with observations.
- Arctic Sea Ice: Analyzed in relation to GCM outputs.

Natural variability cannot explain modern warming since approximately 1900.
- Unforced variability (e.g., ENSO) has some uncertainty but appears too small and variable to be responsible.
Emissions of $CO_2$ and other GHGs can explain the warming.
- The physics of GHG-caused warming are well-established.
- Strong correlation between $CO_2$ and temperature in paleoclimate records provides evidence that GHGs can alter climate.
- Scientists predicted in 1896 that $CO_2$ emissions from fossil fuel combustion would warm the Earth.
- Sophisticated GCMs match the observed warming only when anthropogenic forcings are included.
IPCC attribution conclusions:
- Extremely likely that humans are the main driver of the observed warming.
- Likely that humans are responsible for all the observed warming.

Gallup polls on climate change are conducted every March.
The top graph addresses which question relates to attribution.
- Do Americans think climate change is anthropogenic?
- What is the scientifically supported answer?
The bottom graph explores why the percentage of Americans who believe climate change is caused by humans has remained roughly constant for the last 20 years.
- Margin of sampling error: ± 4% at 95% confidence interval.
- Belief in Human Responsibility.