September 2009 

State of the Science FACT SHEET

Aerosols and Climate 

This document represents the state of the science as described by NOAA 

researchers. NOAA’s Aerosol-Climate activities include targeted research to 

understand the processes that control atmospheric aerosol distribution and 

properties, monitoring to assess how aerosols are changing over time, and 

modeling to quantify how aerosols affect climate. 

What are the Impacts of Aerosols on Climate? 

Aerosols are suspensions in air of microscopic solid or liquid 

particles that profoundly affect Earth’s climate even though their 

mass in the atmosphere is nearly a hundred thousand times less 

than that of CO2. They mostly reflect sunlight and thus cool Earth’s 

surface. However, a fraction of aerosols, black carbon, has the 

opposite effect of absorbing incoming light and warming the 

atmosphere. The scattering of light by aerosols is visible as haze. 

These interactions with incoming solar light are referred to as 

“Direct Effects.” 

Aerosols also modify cloud amount, cloud distribution, and cloud 

brightness. Aerosols may change the amount, timing, and location 

of rain and snow through their influence on cloud properties and 

cloud amounts. These effects are referred to as “Indirect Effects.” 

The net contribution of the direct and indirect effects is to cool 

Earth. They partially counter the effects of greenhouse gases that 

heat Earth’s surface and lower atmosphere. The cooling by 

human-induced aerosols offsets significantly the heating by 

human-induced greenhouse gases. 

The uncertainty in the expected global temperature response 

resulting from various climate change agents is due in part to 

uncertainty in aerosol properties, distribution, and effects on 

clouds. 

Figure 1. The calculated globally averaged radiative forcing of the atmosphere 

due to greenhouse gases (red), and aerosols (blue) (adapted from IPCC AR4 for 

the year 2005). On a global scale, the cooling by aerosols has partially offset the 

warming by greenhouse gases. However, the uncertainty in the contribution to 

cooling is very large. Further, the radiative forcing by aerosols varies greatly in 

space and time. 

Figure 2. Global annual-mean surface temperature change: measured data 

(black line), model results with greenhouse-gases only (blue line), and model 

results with greenhouse gases and aerosols (red line is the average while the 

yellow shows the range of results). Including aerosols in climate models is 

essential to successfully account for the observed overall temperature increases 

and the temporal trends. The influence of aerosols is much more pronounced on 

regional scales. The effect of volcanic aerosols, which last for a short period, are 

clearly visible following major volcanic eruptions that are shown as vertical gray 

bars.

Why are there Uncertainties in the Role of Aerosols in 

Climate Change? 

There are many challenges in understanding the role of aerosols 

in climate change: 

• Aerosols consist of many different chemicals. Their 

composition and size affect how they absorb or scatter light, 

how much water vapor they absorb, how effectively they act to 

form cloud droplets, and how long they reside in the 

atmosphere. 

• Unlike greenhouse gases, which are fairly uniform over the 

entire Earth, aerosol composition and concentrations vary 

tremendously in space and time. 

• Aerosols can both warm and cool Earth. Black soot warms the 

Earth; white haze cools it. The amount of heating or cooling 

depends on whether the particles are over land, water (a dark 

surface), or snow (a white surface) and whether or not clouds 

are present. The warming and cooling may be large enough to 

affect regional weather patterns. 

• Aerosols have multiple effects on clouds. They make some 

clouds brighter, may make clouds last longer, and may make 

some clouds evaporate more quickly. 

• Aerosols affect clouds on local scales. Examples include “ship 

tracks” that form when ship exhaust modifies clouds over the 

ocean. Scientists do not yet understand how all the different 

local effects add up on the global scale. 

• Aerosols can change the way cloud droplets grow to form 

snow or rain. However, scientists still do not understand all 

aspects of this process well enough to make firm predictions 

about how precipitation is affected by aerosols.

Aerosols and greenhouse gases affect Earth’s climate. 

However, our understanding of the aerosols’ effect on climate 

is much less certain than that of greenhouse gases. 

Improving our understanding of the impacts of aerosols on 

climate is essential for providing accurate climate forecasts 

and the information needed to cope with (adaptation) and 

reduce (mitigation) climate change.
 September 2009 

 

Figure 3. Highly diverse sources of atmospheric aerosols (small images). 

Aerosols are emitted into the atmosphere both naturally (green circles) and 

as a result of human activities (red circles). They also are created and 

modified by chemical processes in the atmosphere (blue circles). These 

sources vary regionally, leading to highly variable amounts of aerosols in 

different parts of the globe, as shown in the middle circle (pollution aerosol 

plume over the North Atlantic from the U.S.; and the natural Saharan dust 

over the tropical Atlantic). 

How is NOAA Addressing Aerosol-Climate 

Interactions? 

Aerosols are currently the least understood driver of climate 

change. Understanding and quantifying the role of aerosols in 

climate change are essential for judicious policy decisions and 

are being addressed by NOAA through various activities: 

• NOAA develops state-of-the-art instruments to measure 

aerosol concentrations and properties, and utilizes these 

instruments to investigate aerosols in the laboratory and 

the atmosphere. 

• NOAA works with national and international agencies to 

improve the accuracy and reliability of aerosol 

measurements. 

• NOAA measures aerosol particles at a number of 

monitoring sites to enhance understanding of trends in 

aerosol concentrations and properties as well as aerosol 

sources and sinks. Small aircraft profile the atmospheric 

aerosol to obtain information on heating/cooling by 

aerosols at different heights. 

• NOAA engages in large field studies with aircraft and 

ships to enhance understanding of the processes that 

affect aerosol distribution (sources, atmospheric transport, 

chemical transformation, and deposition) and properties 

(amount of light scattered or absorbed, cloud nucleating 

ability). These studies, together with the long-term 

monitoring that provides a continuous record, allow testing 

of climate models and validation of satellite-derived 

aerosol products. 

• NOAA uses operational satellites to monitor global aerosol 

concentrations and biomass burning and forest fires. The 

data collected are analyzed to detect changes in aerosol 

over various temporal and spatial scales, to characterize 

fire-related aerosol sources, and to investigate feedback 

between fire activity and other climate factors. 

• NOAA develops sophisticated computer models to study 

the effect of aerosols on clouds, the reflectivity of the 

planet, and climate. These models range in scale from 

high-resolution models of cloud systems to regional and 

global models. 

• NOAA uses laboratory, field, and modeling studies to 

characterize the role of aerosol particles in influencing 

precipitation. This issue will be of increasing importance in 

a projected warmer climate as rainfall patterns are 

expected to change and the frequency of droughts and 

flooding is expected to increase. 

• NOAA synthesizes observations and model results to 

assess uncertainties in aerosol distributions, physical and 

chemical properties, and climate effects by a concerted 

effort from aerosol, cloud, and radiation scientists, using 

observations and theoretical calculations. 

• NOAA’s air quality program addresses the air quality 

aspects of aerosols because of the effects of aerosols on 

human health. 

• NOAA synthesizes information in user-friendly terms for 

use by decision makers.

Resources for Additional Information: 

OAR Earth System Research Laboratory and Pacific 

Marine Environmental Laboratory – Intensive field and 

laboratory studies to enhance understanding of processes that 

affect aerosol properties. Long-term monitoring networks to 

provide a continuous record of aerosol properties. 

http://www.esrl.noaa.gov; http://www.pmel.noaa.gov

OAR Geophysical Fluid Dynamics Laboratory – 

Development and use of climate models to study long-term 

climate variability and change and to provide multi-decadal 

climate projections. http://www.gfdl.noaa.gov

NOAA Climate Program Office – Competitive research 

support for developing a predictive understanding of the 

climate system and observational capabilities required for 

advancing climate research. http://www.climate.noaa.gov

OAR Air Resources Laboratory – Intensive field studies to 

improve the understanding of the transport and deposition of 

aerosols and collaboration on the design of regional climate 

models. http://www.arl.noaa.gov 

NESDIS Center for Satellite Applications and Research –

Development and use of long-term records of satellite 

observations to study aerosol variability and its effects on the 

radiation balance http://www.star.nesdis.noaa.gov; creation of 

long-term fire datasets from geostationary observations 

http://cimss.ssec.wisc.edu/goes/burn/wfabba.html; and 

development of enhanced fire monitoring capabilities from next 

generation operational geostationary and polar orbiter 

satellites.