Climate Analogs & Cryosphere

PNAS December 10: Models Show Abrupt Transition of Climate to Past Geologic States

Main Messages:

• Already in 2030, Earth’s climate will approach that of the mid-Pliocene (-3 million years) if current emissions levels continue unabated, eg RCP8.5 through 2030.
  • Pliocene (–3 million), temps 2.8°-4.6° above pre-industrial, with 20+ m. SLR
  • If emissions continue on an RCP8.5 pathway, the global climate reaches an early Eocene state already by 2150.
  • Eocene (–50 million), temps +14° pre-industrial, no ice (Arctic occupied by swampy forests, either no or very small Antarctic ice sheet).
• If emissions instead track current NDCs (eg RCP4.5), climate still reaches a mid-Pliocene state around 2040, but stabilizes at that new state.
  • The accelerated rate of change appears to be faster than anything that has occurred globally in the past, even on geologic time scales.

Background:

Scientists from the University of Wisconsin worked with colleagues at the University of Bristol, Columbia University, University of Leeds, NASA Goddard and NOAA to examine similarities between RCP4.5 and RCP8.5, and several periods of geologic history. These included the Early Eocene, the mid-Pliocene, the Last Interglacial (–129-116,000 CE), the mid-Holocene (6,000 years ago), pre-industrial, and the mid-1900’s.  They used three models: Hadley (HadCM3), NASA-GISS and CCSM.

Across each model, compared to previous eras the Earth’s climate fairly rapidly diverged from recent eras and most closely resembled the mid-Pliocene already by 2030 (under RCP8.5) or by 2040 (under RCP4.5). Under the greenhouse gas stabilization scenario of RCP4.5, the climate then stabilizes at mid-Pliocene-like conditions; but under the higher greenhouse gas emissions of RCP8.5, the climate continues to warm until it begins to resemble the Eocene in 2100, achieving Eocene-like conditions more broadly by 2150.

According to the models, under RCP8.5 these deep-geologic-time climates emerged first from the center of continents, and then expanded outward over time.  Temperatures rise more quickly in these central regions.  Towards 2200, climates become temperate even near the Earth’s poles.

The study also showed that under RCP8.5, “novel” climates emerge across nearly 9 percent of Earth’s land areas. These are conditions that do not have geologic or historical precedent among the time periods studied, and concentrate in eastern and southeastern Asia, northern Australia and the coastal Americas.

Implications of this paper: These findings make even more evident the SR1.5 conclusion that emissions must peak and begin following lower-emissions pathways from current levels within the next 12 years, by 2030; and show the climate changing much more abruptly than previously thought.  Note that the Pliocene saw levels 20+ meters SLR above those of today.  “Stabilization” even at RCP4.5 levels will therefore pose severe challenges to humanity’s ability to adapt without widespread disruption and migration/managed retreat.