What has become clear from the science is that we cannot burn all of the fossil fuels without creating a very different planet.

James E. Hansen, the longtime NASA climate scientist who helped bring global warming to public attention (notably with his 1988 U.S. Senate testimony), has repeatedly argued that the carbon contained in coal, oil, and gas far exceeds what the climate system can safely absorb. The quote reflects a central conclusion of late-20th- and early-21st-century climate science: if humanity exploits and burns essentially all recoverable fossil-fuel reserves, atmospheric CO₂ would rise to levels associated with major, long-lasting changes in temperature, sea level, and ecosystems. Hansen has used this framing in public lectures, interviews, and advocacy to emphasize that the issue is not merely incremental warming but the risk of pushing Earth into a qualitatively different climatic state.

The statement compresses a scientific risk assessment into a moral and political warning. “Cannot burn all” does not mean physically impossible; it means doing so would predictably produce consequences incompatible with a stable, familiar Holocene-like climate. “A very different planet” signals nonlinear, long-lived impacts—ice-sheet loss and sea-level rise, shifting climate zones, intensified extremes, and widespread ecological disruption—that would persist for centuries to millennia. The quote also reframes climate change as a budget problem: the limiting factor is cumulative carbon emissions, not short-term annual fluctuations. Implicitly, it argues for leaving a substantial fraction of fossil fuels unburned and accelerating a transition to low-carbon energy.