research

Paleo climate dynamics:

Why paleo...? An interest in past climate dynamics may seem esoteric to the uninitiated, especially to students with physics background, and in particular interest in events that have occurred tens of millions of years ago. Still, there are some very good reasons to be interested in past climate processes. First, there is the good old "understanding the past is a necessary condition for being able to correctly predict the future". Seems banal, but this turns out to be a surprisingly relevant motivation. Second, and perhaps no less importantly, it turns out that the geologic past contains wonderful climate mysteries of dramatic events which are still not understood and pose challenging puzzles waiting to be solved.
Our work on paleo climate may be divided into several areas,

Glacial cycles,
Abrupt climate change (DO and Heinrich events),
Equable (warm) climates (146-34 Myr ago, and implications to future global warming)

Glacial cycles: during the past 800,000 years or so, the earth went through dramatic ice ages every 100,000 years. During each of these events large parts of North America and Europe were covered by up to 3km high ice sheets, sea level dropped by 130 meter, and the co2 concentration dropped by some 30%. These events are by far the largest climate signal over the past 1 million years, and we still do not understand what caused them. A possible mechanism for these glacial-interglacial oscillations was proposed (with Hezi Gildor) which we call the "sea-ice switch mechanism": a rapid growth or melting of sea ice cover in the northern polar oceans is proposed to shift the global climate system from a growing land-glacier mode (glaciation) to a withdrawing glacier mode (deglaciation). [e.g., 1, 2]

The land ice volume (upper panel) and sea ice area (lower) as function of month and time during the glacial cycle from a model of the coupled ocean, atmosphere, land ice and sea ice components of the climate system.

Abrupt climate change: ice core records from Greenland show that climate was very unstable during the last ice age. "DO events" occurred every 1500 years or so, involving abrupt warming events by 10 degree C within 20 years, lasting a few hundred years and followed by abrupt cooling. "Heinrich events" occurred every 7-10,000 years, and involved massive collapses of ice from the land into the ocean, with a significant climate impact around the North Atlantic. We proposed that sea ice is likely to be involved in these events, strongly amplifying the climate signal created by changes in the ocean thermohaline circulation. [e.g., 1, 2, 3]

The response of an atmospheric general circulation model to changes in surface sea ice cover, demonstrating that sea ice has a dramatic effect which is likely the explanation for the DO warming events observed in the Greenland ice cores.

Equable climate dynamics: The climate of the Cretaceous and Eocene (146-34 Million years ago) was exceptionally warm. Crocodiles and Palm trees, which cannot withstand a night of sub freezing temperatures, could be found in the waters of Greenland and in the middle of present day Canada, were current winter temperatures can drop to -40C. State-of-the-art climate general circulation models cannot reproduce the exceptionally warm continental winter temperature during these periods even with very high atmospheric CO2 concentrations. One wonders whether these models are missing some significant feedback that may also affect their future global warming predictions.
We proposed (with Dorian Abbot) that proxy observations of warm high latitude temperatures and low equator to pole temperature difference during the Eocene and other similar equable climate periods can be explained by tropical-like deep atmospheric convection at high-latitudes. The radiative effects of the high tropospheric clouds associated with the atmospheric convection act to keep the surface warm, and this in turn maintains the convection active. This positive cloud feedback activates as the CO2 concentration is increased, and can explain the absence of ice at high latitudes during the winter as well as the small amplitude of the seasonal cycle. These ideas are developed and investigated using a hierarchy of models: from analytically solvable models, via a simple zonally-averaged two-level atmospheric model (the NCAR Single Column Model with state-of-the-art atmospheric physics, high vertical resolution, a full seasonal cycle, a thermodynamic sea ice model, and a mixed layer ocean) and finally 3D state-of-the-art IPCC coupled ocean-atmosphere-sea ice models used for global warming studies. [1, 2]

Two state-of-the-art coupled climate models demonstrating the convective cloud feedback in a future greenhouse warming scenario. Shown are atmospheric variables over the arctic for the GFDL model (bottom) and NCAR model (top), both at CO2=1120 ppm, and averaged over the polar night winter months (Dec, Jan, Feb). The NCAR model shows (from left to right) a complete arctic sea ice melting, surface temperature increasing up to 25-30C, large cloud radiative forcing, and increased convective precipitation, showing that atmospheric convection is active, leading to the clouds, cloud radiative forcing, surface heating and sea ice melting. The GFDL model shows this feedback active only over a small part of the arctic. Somewhat increased CO2, or longer integration period, are very likely to lead to this feedback also showing up over the entire arctic in the GFDL model. The convective cloud feedback originally proposed to explain the Eocene climate therefore seems also relevant for high CO2 future global warming scenarios.