Climate change

Setting of experiments

Idealized geography, symmetric w.r.t equator. No seasonal cycle.

- A: Standard exp. Present CO
_{2}concentration (300 ppm) is kept. Look at the quasi-steady state. - B: Steady-state response exp. 4-times the present CO
_{2}concentration (1200 ppm) is kept. Look at the quasi-steady state. Difference "B - A" is considered as "steady-state response to CO_{2}quadrupling". (From experience of previous numerical experiments, response to 4xCO2 is nearly twice of response to 2xCO2. The authors used larger forcing in order to make signals more visible.) - C: Transient response exp. to step-like forcing.
At one time in the quasi-steady-state of A,
CO
_{2}concentration is quadrupled suddenly, and kept at the new level.

The result of C generally looks like approaching the state of B according to an exponential function of time (with a negative coefficient).

Result: Meridional (South-North) - vertical section of temperature difference

B (steady state exp.) - A (standard exp.)

- Stratosphere cools.
- Troposphere and ocean warm.
- In troposphere, the lower part in high latitudes warms strongly. Because ...
- In cold climates, "near-surface temperature inversion" (dT/dz > 0) is often observed. In warmer climates, this situation occurs less often.
- There are less snow and sea ice in warmer climates. (Ice-albedo feedback)
- Warmer climates have more condensation of water vapor, resulting in smaller lapse rate (-dT/dz).
- (These facts already appear in Manabe & Wetherald 1975, where just the heat capacity of the mixed layer (～100 m) was considered as the ocean's role.)

- The upper ocean warms toghether with the troposphere at the same latitude.
- The deep ocean warms together with the lower troposphere in the high latitude, where the sinking water forms.

Result: Meridional (South-North) - vertical section of temperature difference

C (25 years after CO_{2} quadrupling) - A (standard exp.)

- Temperature of deep ocean hardly changes yet in 25 years.
- Temperature changes in the atmosphere and the upper ocean is similar to the steady-state response, and the magnitude is approx. 70 % of s.s.r.

The delay of global warming due to ocean heat capacity
can be approximated by a linear model
with a time constant corresponding to
the heat capacity of the **upper ocean**
(the part of ocean where wind-driven circulation dominates).
The time series can be approximated by an exponential function
tending to the new steady state.

A rough estimate of future temperature can be made
by giving a realistic scenario of CO_{2} concentration
to the linear model.
(More detailed estimate will be done by
giving such a CO_{2} concentration scenario
to coupled 3-dimensional climate models.

2019-June-06

MASUDA Kooiti