Evidence for Climate Change

Ice cores: not a proxy for the gas, a sample of the gas

As snow compacts into glacial ice it traps air bubbles. Drilling at Vostok and EPICA in Antarctica has recovered a continuous record back roughly $800{,}000$ years. The bubbles are actual ancient atmosphere, so carbon dioxide and methane concentrations are measured directly rather than inferred. Temperature comes from a separate proxy in the same core: the ratio of heavy to light isotopes of oxygen and hydrogen in the ice, because colder air delivers isotopically lighter snow.

Over eight glacial cycles carbon dioxide oscillated between about $180$ ppm in glacial maxima and about $280$ ppm in warm interglacials, and temperature tracked it closely. The present value, above $420$ ppm, lies far outside that entire $800{,}000$-year envelope.

The instrumental and satellite records

• Keeling curve — continuous carbon dioxide measurement at Mauna Loa since $1958$, rising from $315$ ppm to over $420$ ppm, with an annual sawtooth of $6$–$7$ ppm caused by Northern Hemisphere plants taking up carbon each summer.
• Surface temperature — about $1{,}2\ ^\circ\text{C}$ of warming since the late $1800$s, with the fastest rise in the Arctic.
• Sea level — roughly $1{,}7$ mm per year through the twentieth century, now about $3{,}4$ mm per year from satellite altimetry.

Sea level rises from two independent causes: thermal expansion of seawater already in the ocean, and the addition of meltwater from ice resting on land. Floating sea ice already displaces its own weight, so its melting does not change sea level.