The climate system is comprised of five natural components:
- the atmosphere,
- the hydrosphere,
- the cryosphere,
- the land surface, and
- the biosphere.
The atmosphere is the envelope of gases that surrounds Earth, including the naturally occurring greenhouse gases that warm the planet’s surface. The hydrosphere includes all of Earth’s liquid water and gaseous water (water vapor), whereas the cryosphere includes all frozen water (ice). Note that the cryosphere is technically part of the hydrosphere, but climate scientists usually treat it as a separate component of the climate system because its physical properties differ from those of water and water vapor. The land surface does not include water- or ice-covered surfaces but consists of all other vegetated and non-vegetated surfaces. The biosphere is the realm of life and is found in all of the other natural components, especially the hydrosphere and land surface. In fact, the biota is made up of and requires the presence of air, water, and mineral matter—that is, material from the atmosphere, hydrosphere, and land––to exist. Several external forces influence the five climate system components, with radiation from the Sun being most important. Climate scientists consider the impact of human activities on the climate system another example of external forcing.
The framework in Figure 1.2 shows how people interact with the climate system. Starting on the left-hand side, human activities, such as land clearing and fossil fuel burning, put heat-trapping greenhouse gases into the atmosphere, thereby changing the atmosphere’s composition, increasing the so-called greenhouse effect, and warming the near-surface layers of the atmosphere. These human activities are therefore causing climate change (top center), which has many characteristics beyond surface warming, including increased evaporation, changed rainfall quantity, intensity, and location, decreased ice and snow cover, and increased sea level among others. These climate changes have impacts on physical systems, biological systems, and human systems (right-hand side), with most of these impacts being negative. People respond to these impacts in two ways (bottom), either through mitigation or adaptation. Mitigation aims to reduce or eliminate the causes of climate change; adaptation seeks to reduce or eliminate the impacts. Together, the impacts of and responses to climate change make up the total consequences of climate change. We will expand on the human causes and consequences of climate change later in this lesson and later in this course.
The human dimensions of climate change shown in Figure 1.2 interact at all scales of the climate system and human activity. The human causes of climate change result from billions of daily local actions––such as emissions of carbon dioxide from fossil fuel combustion and forestry––that accumulate to cause a change of global climate. This global-scale change plays out differently in different regions, warming most areas while wetting some areas and drying others. These regional climate changes lead to local impacts that have more or less severity depending on the vulnerability of each place’s natural and human systems. Responses vary, too, with local, regional, and global efforts both to mitigate greenhouse gas emissions and adapt to climate change.