|Special Topic: Global Environment and Atmospheric
|Future Projection of Global
- Toward Understanding Chemistry-climate
||Future prediction experiments of global atmospheric
pollution are conducted with CHASER, a chemistry coupled climate
model. The experiments show that future distributions of tropospheric
ozone are largely influenced by climate change (global warming),
as well as by increases in emissions of pollutants (precursor
gases). Here introduced is a challenge toward climate change
prediction in the context of chemistry-climate interaction.
Atmospheric Composition Research Program
Frontier Research System for Global Change (FRSGC)
Chemistry Coupled Climate Model
Ozone in the troposphere, one of the significant greenhouse
gases, controls concentrations of other pollutants and greenhouse
gases such as methane and halofluorocarbons through chemical reactions.
Tropospheric ozone chemistry also affects formation of sulfate aerosol,
which plays a significant role for climate change and causes acid
rain. Since the most of tropospheric ozone is produced associated
with air pollution (emissions of ozone precursors), we need to assess
the impacts of future pollution on climate and atmospheric environment,
especially in Eastern Asian regions, where significant economic
development is expected.
For such purpose, we have developed a chemistry coupled
climate model CHASER*1to simulate tropospheric ozone
chemistry and its impacts on climate. The CHASER model is also used
for short-term forecast in the "chemical weather forecasting
system". Using this model, we have started predicting future
changes in ozone and other pollutants distributions and their impacts
Significant Roles of Climate Change in Future
Prediction of Air Pollution
We perform future simulations of important pollutants
such as ozone and sulfate using CHASER with the IPCC scenarios.
In the simulation which considers changes in emissions of ozone
precursors (nitrogen oxides, carbon monoxide, etc.) only, significant
increases in surface ozone are calculated in eastern Asia in response
to enhanced chemical production of ozone with the expected emission
increases (e.g., Fig. 1). Our simulations also show that enhanced
ozone production in the upper troposphere over eastern Asia has
a huge impact on future global distributions of ozone owing to rapid
intercontinental transport out of the region.
However, our recent study suggests that these changes
in tropospheric ozone associated with emission changes can be modulated
by future climate change (warming). Fig. 2b shows the differences
in the simulated zonal mean ozone distributions between with and
without climate change for 2100. With climate change, lower tropospheric
ozone levels are reduced due to increased chemical loss of ozone
associated with water vapor increases. Upper tropospheric ozone
in the high latitudes also decreases reflecting the rises in the
tropopause height induced by climate change. On the other hand,
the model shows increases in upper tropospheric ozone in the low-mid
latitudes due to climate change. These ozone increases are attributed
to the enhanced ozone input from the stratosphere (Fig. 3) resulting
from enhancement in the stratospheric and tropospheric circulation
with climate change. These results imply that for prediction of
future ozone pollution, it is necessary to take into account the
feedbacks from climate change as well as emission changes.
|Fig. 1 Surface ozone increases (ppbv) predicted for
2050 and 2100 with the IPCC SRES-A2 emission scenario. Surface ozone
levels in eastern Asia are simulated to increase by ~50% in 2050 and
by ~100% in 2100 relative to the present-day.
|Fig. 2 Annual and zonal mean ozone distribution predicted
for 2100 with the A2 scenario in the control experiment (with emission
changes only) (a); climate change (warming) induced changes in the
ozone distribution (shown as the warming experiment minus the control
experiment) for 2100 (b). In (b), the zonal mean tropopause is also
shown for 2100 (solid line) and 1990 (dashed line).
|Fig. 3 (Upper panel): temporal evolution of net stratospheric
ozone influx to the troposphere for 1990-2100 calculated in the climate
change (warming) and control experiment.(Lower panel): global and
annual mean surface air temperature change relative to 1990 in the
warming experiment. The decreases in net stratospheric ozone input
in the control experiment are due to increases in tropospheric ozone
associated with the emission increases.
Need for the Future Prediction in the Framework of Earth System
Ozone, while it is one of the important factors affecting climate,
it also has significant influence on climate by chemically interacting
with methane and aerosols. In addition, this interaction is subject
to some changes with climate change as discussed above*2.
It is therefore necessary to understand air pollution and climate
as one system. Using the chemistry coupled climate model, we are working
on prediction of future changes in climate and atmospheric environment
in the context of chemistry-climate interaction. Also, the CHASER
model is expected to play an important role in the integrated Earth
system modeling at FRSGC which is part of the "MEXT's (Ministry of
Education, Culture, Sports, Science and Technology) Project for Sustainable
Coexistence of Humans, Nature, and the Earth".
||CHASER simulates production and distributions of pollutants
such as ozone and sulfate, considering emissions of precursors
(NOx, CO, SO2, etc.), transport, chemical
reactions, and deposition in the CCSR/NIES/FRSGC climate model.
||Our future simulations as discussed here also show that temporal
evolution of methane and aerosols is highly dependent on future
changes in ozone and climate.
Sudo K., M. Takahashi, and H. Akimoto., Future changes in stratosphere-troposphere
exchange and their impacts on future tropospheric ozone simulations,
Geophysical Research Letter, Vol.30, No.24, 2256, Doi:10.1029/2003GL018526,