The consensus of the world’s climate scientists, as generated by the UN-sponsored Intergovernmental Panel on Climate Change (IPCC), is that:

Question

BlackTom AI · Accepted Answer

Greenhouse gases such as CO2 absorb outgoing infrared radiation and thereby warm Earth’s surface; this basic physics underlies the IPCC’s findings about anthropogenic climate change.

1) Relevant knowledge points and concepts
- Greenhouse effect: CO2 and other greenhouse gases trap heat, raising global average temperatures.
- Attribution: Multiple lines of evidence (observations, physics, climate models) attribute recent global warming primarily to increased greenhouse gas emissions from human activities since the Industrial Revolution.
- Projections: Climate models and IPCC assessments show that continued high emissions (business-as-usual) will produce substantially larger temperature and climate changes over the coming centuries than the modest changes observed so far.
- Ozone depletion is caused by chlorofluorocarbons (CFCs), not CO2; methane has generally increased from human sources (e.g., agriculture, fossil fuels), not decreased.

2) Step-by-step reasoning
- Observations show rising atmospheric CO2 concentrations since the 1800s, measured in ice cores and direct sampling.
- Radiative physics demonstrates that increased CO2 raises radiative forcing, which models translate into global temperature increases.
- IPCC synthesis of observational evidence and model simulations concludes humans are the dominant cause of recent warming.
- Model projections under high-emission scenarios produce much larger warming and associated impacts (sea-level rise, extremes) over the next centuries than have occurred to date, unless emissions are reduced.

3) Intermediate steps / evidence
- Measured CO2 increase: preindustrial ~280 ppm to >410 ppm (observational fact underpinning attribution).
- IPCC statements explicitly note anthropogenic greenhouse gas emissions are the main driver and that future changes will be larger without mitigation.

4) Why the correct answer is correct
- The third option summarizes the IPCC consensus: human-caused CO2 increases have warmed the planet and current changes are small relative to projected future change if behavior is not altered.

5) Why other options are incorrect
- Option 1: Links CO2 to the ozone hole—incorrect; ozone depletion is due to CFCs.
- Option 2: Claims future changes will be smaller than past ones—contradicted by IPCC projections.
- Option 4: Claims humans have lowered CO2—false; CO2 has risen.
- Option 5: Nonsensical claim about lowered methane and cow flatulence—false and not an IPCC conclusion.

6) Conclusion
Therefore the correct choice is: "Human activities have raised CO2 levels in the atmosphere, warming the planet, and the changes so far have been small compared to the changes that are likely over the next centuries unless we humans alter our behavior." This matches the provided answer.

The consensus of the world’s climate scientists, as generated by the UN-sponsored Intergovernmental Panel on Climate Change (IPCC), is that: 单项选择题

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