2010 March 16: U.S. House of Reps. Hearing: Testimonial by V. Ramanathan on Climate Impacts of Black CarbonClimate

 

2010 March 16: U.S. House of Reps. Hearing: Testimonial by V. Ramanathan on Climate Impacts of Black CarbonClimate

 

Impacts of Black Carbon

V. Ramanathan

Scripps Institution of Oceanography

University of California, San Diego

*Testimonial to the House Select Committee on Energy Independence and Global Warming

Chair: The Honorable Edward J. Markey

Ranking Member: The Honorable F. James Sensenbrenner, Jr

House Hearing entitled:

Clearing the Smoke: Understanding the Impacts of Black Carbon Pollution

March 16, 2010

2175 Rayburn House Office Building

Washington D.C.

1

My Background

I am an atmospheric physicist. My work on black carbon and its radiative warming is largely

based on experimental and observational studies. I Co-Chaired the Indian Ocean Experiment in

the 1990s which looked at long range transport of black carbon from S Asia and its regional

radiative forcing. I also chair the United Nations Environmental Program’s Atmospheric Brown

Clouds (ABC) project and set up regional observatories in the Arabian Sea, Nepal and western

Pacific to observe long term variations in black carbon and other manmade particles. With

researchers in my group, I have developed autonomous unmanned aerial observing systems

(UAS) with miniaturized black carbon and radiometers instrumentation to measure directly how

black carbon and other aerosols modify the heating of the atmosphere. UAS campaigns have

been conducted in the Arabian Sea, in S California and in the western Pacific during the Beijing

summer Olympics to examine the impact of the ‘great pollution shutdown’ in Beijing.

ABC Observatories Indian Ocean Experiment

Unmanned Aerial Vehicle Observing Systems

2

I. SYNOPSIS

This testimony is largely based on a synthesis article published in 2008: Ramanathan, V. and G.

Carmichael (2008).

What is Black Carbon(BC)?

: Black carbon

is the particle (also known as Aerosol)

which gives the darker color to smoke from

diesel vehicles or fires. BC is generated

through cooking with solid fuels (wood, cow

dung, crop residues), by bio mass burning

(savanna burning, forest fires and crop

residue burning) and fossil fuel combustion

(diesel, solid coal and others).

Atmospheric Brown Clouds (ABCs)

: In the

atmosphere, BC is mixed with other

particles such as sulfates, nitrates, dust and

other pollutants, and together, the mix of

manmade particles are sometimes referred to

as Atmospheric Brown Clouds (ABCs). The

name “Brown Clouds” is due to the fact that

the mixture of BC and other aerosols gives a

brownish color to the sky.

Physics of Climate Warming Effects by BC:

BC is one of the strongest absorbers of solar

radiation in the atmosphere and thus it is a

source of global warming. In addition to BC,

smoke also contains some organic aerosols

which also absorb visible and UV solar

radiation and such organic aerosols are

called as Brown Carbon. Black carbon is

removed from the atmosphere by

precipitation. When BC is deposited on

snow and ice, it darkens them which in turn

increases absorption of sunlight by snow and

ice. This darkening effect contributes to

surface warming of the arctic and the alpine

glaciers.

Observationally Constrained Estimate of

Heating: The estimate of BC heating by this

author’s group is constrained by ground

based, aircraft and satellite observations. We

estimate that the current (2000-2003) global

warming effect of BCs may be as much as

60% of the current (2005) CO

2 greenhouse

warming effect. Most model based estimates

of BC warming effect are smaller and are in

the range of 20% to 50%.

Global Water Budget:

Digressing to all

particles in ABCs, ABCs enhance scattering

and absorption of solar radiation and also

produce brighter clouds that are less

efficient at releasing precipitation. The net

result is a large reduction of sunlight at the

surface, popularly known as dimming. The

interception of sunlight in the atmosphere by

BC and the surface dimming, along with the

micro-physical effects can lead to a weaker

hydrological cycle and drying of the planet.

ABCs and black carbon are thus linked with

the availability of fresh water, a major

environmental issue of the 21st century.

Regional Climate Impacts:

The regional

effects of BC are estimated to be particularly

large over Asia, Africa and the Arctic. Since

the dimming and atmospheric heating are

non-uniform in space and time, BC leads to

changes in north-south and land-ocean

contrast in surface temperatures, in turn

disrupting rainfall patterns. For example, the

Sahelian drought, the decrease in the

monsoon rainfall over India and the drying

of northern China are attributed by models

to BC and other aerosols in ABCs. Recent

studies employing unmanned aerial vehicles

showed that BC enhances atmospheric solar

heating by about 25% to 50% in S.Asia, E.

Asia and in California. Model studies

suggest this heating to have contributed as

much as greenhouse warming to the large

warming observed over elevated regions of

the Himalayan-Tibetan glacier region. In

addition, the deposition of BC over the

bright snow and ice surfaces darkens these

3

BC from Fires Heats the Air

0

1000

2000

3000

4000

0 5 10 15 20 25 30

Abs coeff

Altitude (m)

880 nm

525 nm

370 nm

370 BC only

525 BC only

California Wildfires Of July 2008:

BC; BrC and Radiative Heating

Corrigan, Ramana, Ramanathan

California Energy Commission Project

BC Surrounds the Himalayas

Color-coded profiles of 532nm backscatter return signal from the CALIPSO satellite lidar

showing the vertical distribution of ABCs. The image shows ABCs surrounding the

Himalayas from both the S and SE Asian side (Source:

Ramanathan et al., 2007b)

BCs and ABCs Can Decrease Rainfall

Trend in observed rainfall from 1950 to 2002. The figure shows the change

in rainfall between 1950 and 2002; It was obtained by multiplying the year

linear trend in mm/day/year by 52 years. The precipitation data is the

Hadley center CRU data (Source:

Mitchell and Jones, 2005)

surfaces. The resulting increase in

absorption of solar radiation is estimated to

be a major source of warming and melting

of the arctic sea ice and the eurasian snow

mass including the Himalayas and the

Tibetan regions.

Status of Current Understanding:

It is

important to distinguish issues that are well

understood from those that require

confirmation. The first definitive study on

the global warming magnitude of CO

2

increase was published 45 years ago and it

required hundreds of model studies by

numerous groups since then to reach the

current level of consensus on the importance

of CO2 to climate change. In comparison,

observational studies on climate effects of

BC were begun in earnest about 15 years

ago. There is reasonable consensus on the

following issues:

Life time of black carbon in the air is of

the order of several days to few weeks.

Fossil fuel combustion, bio fuel cooking

and biomass burning are the sources of

BC.

BC adds solar heating to the atmosphere

and causes dimming at the surface. The

atmospheric solar heating is much larger

than the surface dimming, and as a

result, BC leads to a net warming of the

surface and the atmosphere.

Deposition of BC on sea ice and snow

darkens the surface and leads to more

solar absorption and melting of sea ice

and snow.

Atmospheric Brown Clouds (i.e., BC

and other manmade particles) lead to

dimming at the surface and the global

average effect of this is to decrease

rainfall.

Globally, BC has a net warming effect

on the climate system. The magnitude of

its current warming effect is subject to a

large uncertainty, ranging from about

4

20% to as much as 60% of the warming

effect of CO2 increase since the 1850s.

Rationale for Mitigating BC Emissions:

BC offers an opportunity to reduce the

projected global warming trends in the short

term. The life time of BC in the air is of the

order of days to several weeks. The BC

concentration and its solar warming effect

will decrease almost immediately after

reduction of its emission. Policy makers will

have a unique opportunity to witness the

success of their mitigation efforts during

their tenure. Reductions of BC emissions are

also warranted from considerations of public

health, air quality and regional climate

change.

Other Considerations for Policy Makers:

Unmasking of the Greenhouse Effect:

A blanket keeps us warm on a cold

winter night by trapping the heat from

our body. Likewise, the greenhouse

gases surround the planet like a blanket

and trap the infrared heat generated by

the planet’s surface and the atmosphere.

Black carbon particles enter this blanket

and heats it by trapping sunlight.

Sources that generate BC also co-emit

other particles made of organics, which

act like mirrors on the blanket and cool

the surface by reflecting sunlight. In

addition, some fossil fuels also generate

other mirror like particles such as

sulfates and nitrates. Because of the

concern over sulfate pollution, emission

of SO

2 has come down by 30% to 50%

in developed nations since the 1980s,

thus eliminating their cooling effect.

This unmasking has been

observed as increased sunlight in most of

Europe and USA during the last few

decades, and needs to be offset by

corresponding decreases in BC.

Complementing CO2 Emission

Reductions: CO2 is the major factor (as

much as 55%) contributing to the

enhancement of the greenhouse effect.

At current rate of emission (35 billion

tons per year) and the current growth

rate of 2% to 3%, the manmade

greenhouse effect can double during this

century. BC reductions, even at 50%,

cannot offset the CO2 effect. However,

BC reductions when combined with

reductions in other short lived climate

warming gases, can delay large warming

by few decades and complement CO2

mitigation efforts.

Diesel and Cook stoves are Prime Targets

for Mitigation:

BC generated by diesel

combustion has greater warming potential

than bio-fuel cooking or biomass burning.

This is because diesel generates less of the

cooling organic aerosols. With respect to

biomass fuel cooking, limited studies

suggest that this source is also a net climate

warmer but we need to conduct a careful and

well documented scientific study of the

impact of biomass cookstoves. Towards this

goal, this author along with a team of NGOs

and public health experts, has proposed

Project Surya (http://wwwramanathan.

ucsd.edu/ProjectSurya.html).

Cooking with solid fuels (wood and cow

dung) is a major source of BCs over S Asia

and has major health impacts on women and

children. Surya will adopt a large rural area

of about 50,000 population, in India, and

provide alternate cooking with biogas plants,

smoke free cookers and solar cookers.

Major Source of Uncertainties:

The basic

input data for most models is the inventory

of emissions of BC from various parts of the

world. This has about a three-fold

uncertainty, particularly for Asia, Africa and

S America. The second major uncertainty is

the inter-action of BC and organics aerosols

in clouds. Relying on just observational

work, BC-Cloud interactions seem to have a

net warming effect.

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