The United Nations-sponsored Intergovernmental Panel on Climate Change shared the 2007 Nobel Peace Prize. The information that the Panel has supplied to policymakers includes:

Question

BlackTom AI · Accepted Answer

IPCC Fourth Assessment (2007) concluded that human activities—especially fossil-fuel burning—are the primary cause of the observed rise in atmospheric CO2 and that this rise is very likely driving observed warming and will lead to larger warming if emissions continue.

1) Relevant knowledge points and concepts
- Atmospheric CO2 records (Keeling Curve) show a steady rise since the industrial era.
- Carbon isotopes (decline in 13C/12C ratio) indicate the added CO2 comes from fossil carbon, not volcanic or biological sources.
- Global carbon budget: anthropogenic CO2 emissions exceed the observed atmospheric increase; oceans and biosphere absorb part, leaving the remainder to accumulate in the atmosphere.
- Climate attribution uses models and observations; the IPCC uses probabilistic language (in AR4 “very likely” = >90% confidence).
- Volcanic CO2 fluxes are small compared with human emissions and volcanic aerosols tend to cool climate short-term.

2) Step-by-step reasoning
- Step 1: Observational data show a marked increase in atmospheric CO2 since the late 18th century.
- Step 2: Isotopic signatures of CO2 match fossil-fuel carbon, proving the dominant source is fossil-fuel burning.
- Step 3: The global carbon budget demonstrates that human emissions are large enough to explain the atmospheric rise; natural sinks do not generate a net increase.
- Step 4: Climate models that include anthropogenic greenhouse gases reproduce the observed warming; models excluding human forcings cannot.
- Step 5: The IPCC synthesizes this evidence and assigns high confidence (“very likely”) that human emissions are causing warming that will grow if emissions continue.

3) Intermediate calculations
- No numerical calculation required here; the carbon-budget and isotopic analyses are the quantitative bases that show anthropogenic emissions exceed natural sources and account for the atmospheric increase.

4) Why the correct answer is correct
- It exactly matches the IPCC AR4 conclusion: human fossil-fuel burning is the primary cause of the CO2 rise, and it is very likely causing warming that is likely to become much larger under continued emissions.

5) Why other options are incorrect
- Options asserting explosive volcanism as the primary cause are wrong because volcanic CO2 emissions are much smaller than anthropogenic emissions and isotopic evidence contradicts a volcanic source.
- The option saying human-caused CO2 has no effect contradicts model/observational attribution showing warming from greenhouse gases.
- The option stating warming is “very likely” but “unlikely to become much larger” misstates the IPCC risk: continued emissions are expected to produce larger future warming.

6) Conclusion
This reasoning confirms the correct choice: "The observed rise in atmospheric CO2 levels has been caused primarily by human fossil-fuel burning, and very likely is causing warming of the climate that is likely to become much larger if we continue our current behavior."

The United Nations-sponsored Intergovernmental Panel on Climate Change shared the 2007 Nobel Peace Prize. The information that the Panel has supplied to policymakers includes:Single choice

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