Tropical Pacific stays El Niño-like for centuries after CO2 emissions stop in models

Why it matters

Reaching net zero would slow further change in the tropical Pacific but would not undo it. Rainfall patterns and extreme El Niño events could stay altered for centuries. The results also point to a benefit of stopping emissions earlier, since later cessation gave larger and longer-lasting changes.

What they did

The authors analysed nine Earth system models from the ZECMIP experiment, in which emissions are set to zero after 1000PgC of cumulative emissions. They also used seven 1000-year net zero runs from one model, ACCESS-ESM1-5, branching off the high-emission SSP5-8.5 scenario between 2030 and 2060. They compared these with pre-industrial control runs and a 40-member ensemble. They tracked the west-minus-east Pacific temperature gradient, El Niño amplitude, extreme El Niño frequency, rainfall belts, winds, thermocline tilt and ocean temperatures.

Key findings

• The west-minus-east Pacific temperature gradient weakened in all ZECMIP models during warming. It stayed weakened for decades after emissions stopped. In ACCESS-ESM1-5 it showed no long-term recovery.
• Extreme El Niño events rose from 9.82% of El Niño events in pre-industrial runs to 21.33% under warming in ZECMIP. They stayed at 20.25% after emissions ceased.
• In ACCESS-ESM1-5, extreme events made up 26.3% of El Niño events after net zero in the B2060 run, against 1.73% in the pre-industrial run. The B2045 and B2030 runs gave 17.0% and 16.4%.
• Continued warming of the Southern and deep ocean reduced the north-south temperature gap. This kept the rain belts shifted toward the equator, the trade winds weakened and the thermocline flattened.
• El Niño amplitude changes were non-monotonic and model dependent. A 100-year window showed amplitude staying above the pre-industrial range for centuries in ACCESS-ESM1-5, especially in B2060.

Limitations

• The millennial-scale results and the feedback mechanisms rest on one model, ACCESS-ESM1-5. The size of the changes is likely model dependent, and shared model biases may exaggerate extreme El Niño frequency.
• Only CO2 emission cessation is simulated, with abrupt cessation. Other greenhouse gases and aerosols are not explored, and the ZECMIP runs are too short to separate forced ENSO change from internal variability.

Glossary

• Zonal temperature gradient (ZTG): The sea surface temperature difference between the western and eastern equatorial Pacific. A weaker gradient means a more El Niño-like mean state.
• Net zero: A situation where human CO2 emissions are cut to zero, so atmospheric CO2 can then evolve freely.
• Thermocline: The layer in the ocean where temperature drops quickly with depth. Its tilt along the equator is tied to the trade winds.
• ZECMIP: A model intercomparison project that studies how climate changes after CO2 emissions are set to zero.