Equilibrium climate sensitivity and cloud feedbacks

7 problems

• How strongly have human-made aerosols cooled the climate?

  Open

  Pollution particles reflect sunlight and make clouds brighter, offsetting part of the warming from greenhouse gases. The size of this offset is poorly known, which blurs what past warming says about future warming.

• What caused Earth's record-low reflectivity in 2023?

  Open

  In 2023 Earth reflected less sunlight than ever measured, mainly because of fewer low clouds, which added to the record heat. Whether this came from cleaner ship exhaust, natural swings, or a cloud response to warming is unclear.

• How much do tropical anvil clouds shrink as Earth warms?

  Open

  High, thin anvil-shaped clouds spread out from tropical thunderstorms and trap heat. If they shrink with warming, more heat escapes to space, a cooling effect called the 'iris' hypothesis, but its size is debated.

• What is Earth's equilibrium climate sensitivity?

  Partially resolved

  Doubling $\mathrm{CO_2}$ will warm Earth by somewhere around 2.5 to 4 degrees once the oceans catch up. Narrowing that range has proved hard for decades.

• Can paleoclimate evidence rule out climate sensitivity above 5 K?

  Partially resolved

  Several recent climate models warm by more than 5 degrees for doubled $\mathrm{CO_2}$. Past climates such as the last ice age may show this is too much.

• How strongly do low marine clouds amplify warming?

  Partially resolved

  Low clouds over the oceans reflect sunlight and cool the planet. If warming thins them, the planet absorbs more sunlight and warms further, and how much is the largest single uncertainty.

• Why has the observed warming pattern damped Earth's apparent sensitivity?

  Open

  The eastern tropical Pacific has barely warmed in recent decades, unlike in most climate models, and this has made Earth look less sensitive than it may be. Whether the pattern is caused by the forcing or is natural variability is unknown.