A NASA DC-8 research flight observes marine clouds off the coast of Baja, Mexico during the four season, multi-agency ATom mission. Photo: Sam Hall, NCAR

CSL climate research has two focal points: (1) understanding aerosol interactions in the climate system; and (2) characterizing the emissions, transport, transformations, and distribution of key climate species. CSL climate research results in an increased understanding and quantification of the radiative, chemical, and dynamical processes that influence climate with a goal of reducing major uncertainties in climate models and, hence, improving our understanding of our current climate and confidence in future climate projections.

Aerosols and their interactions in the climate system are the greatest uncertainties in our ability to model the current and future climate system. The role of aerosols in the climate system is multifaceted, requiring sophisticated instrumentation to measure various parameters such as size distribution, mass, aerosol optical depth, composition, scattering, and absorption. Observations and modeling of aerosols are critical in order to quantify the transport (regionally/globally) and distribution of aerosols that absorb solar radiation and warm Earth's atmosphere (e.g., black carbon) and scatter solar radiation and cool Earth's surface (e.g., sulfate aerosols), including delineation of new chemical pathways for marine sulfate aerosol formation. Observations and modeling also contribute to understanding the climate impacts of aerosols that are not yet well understood (e.g., secondary organic aerosols). Aerosols also play an important role in the climate system due to aerosol-cloud interactions. The influence of aerosols on regional cloud formation, extent, optical properties (Earth's radiation budget - ERB), and precipitation is a continuously evolving field of research. Remote and in-situ observations from the surface, aircraft, ships, and satellites in conjunction with models and machine learning are used to improve the understanding of cloud systems and microphysical processes in order to reduce the uncertainty in determining how aerosol-cloud interactions impact the climate system.

CSL climate research also addresses other key climate species including tropospheric ozone, methane, substitutes for ozone-depleting substances (including hydrofluorocarbons, HFCs), and others. These species contribute directly to climate forcing, influence many climate feedback processes, link climate change and air quality, and are areas of current focus for policy formulation. The distribution and transport of tropospheric ozone influences regional and global climate and ozone is an important trace species for testing climate models. Quantification of the emissions and distribution of chemically active greenhouse gases such as methane is critical in understanding regional and global climate change. Laboratory and modeling studies of solvent and refrigerant replacement chemicals are used to estimate their climate impacts. Observations and modeling of water vapor in the upper troposphere and lower stratosphere (UT/LS) increase our understanding of water vapor's abundance and distribution, which is a critical factor in determining the amount of radiation lost to space and thus determining the energy budget of Earth's surface (also addressed under stratospheric research).

CSL climate research is advancing scientific knowledge, contributing to national and international assessments, and is disseminated to stakeholders. CSL was a lead participant in the 4th US National Climate Assessment, the scientific state-of-understanding assessment report for decision-makers.