.The main way humans impact climate is by altering the concentrations of greenhouse gases in the atmosphere. We do this primarily by emitting gases into the atmosphere, particularly carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), sulfate aerosols, CFCs, and water vapor (H2Ol).
Global greenhouse gas concentrations have tended to increase steadily since the mid-1900s, as these graphs show.
Two trends are obvious in the first graph. First, the data generally move in an upward direction over time, indicating that the concentration of CO2 in the atmosphere has increased steadily since at least 1960, when the graph begins. Second, there are also much smaller oscillations that have a cycle of one year, shown in red and in the lower right inset. These regular, smaller-scale ups and downs in global CO2 concentration correspond with the seasons in the Northern hemisphere, where most of the Earth’s vegetative biomass is located. In the spring and summer (May-August) in the Northern hemisphere, new leaves emerge on plants which begin taking in CO2 for photosynthesis, and the atmospheric CO2 concentration drops. Then in autumn and winter when leaves fall, CO2 is released back into the atmosphere, drawing the curve back upward. The overall upward trend of the graph is also influenced by deforestation, or clearing land of vegetation, which results in less biomass available for CO2 uptake. The graph on the right shows that other greenhouse gases are following trends similar to CO2.
Another way of thinking about human greenhouse house gas emissions is how they contribute to radiative forcing, the difference between incoming and outgoing energy from radiation in the climate system. Positive forcing warms the system, because there is more incoming or trapped energy, while negative forcing cools it, because more energy leaves the system than enters it.
Two trends are obvious in the first graph. First, the data generally move in an upward direction over time, indicating that the concentration of CO2 in the atmosphere has increased steadily since at least 1960, when the graph begins. Second, there are also much smaller oscillations that have a cycle of one year, shown in red and in the lower right inset. These regular, smaller-scale ups and downs in global CO2 concentration correspond with the seasons in the Northern hemisphere, where most of the Earth’s vegetative biomass is located. In the spring and summer (May-August) in the Northern hemisphere, new leaves emerge on plants which begin taking in CO2 for photosynthesis, and the atmospheric CO2 concentration drops. Then in autumn and winter when leaves fall, CO2 is released back into the atmosphere, drawing the curve back upward. The overall upward trend of the graph is also influenced by deforestation, or clearing land of vegetation, which results in less biomass available for CO2 uptake. The graph on the right shows that other greenhouse gases are following trends similar to CO2.
Another way of thinking about human greenhouse house gas emissions is how they contribute to radiative forcing, the difference between incoming and outgoing energy from radiation in the climate system. Positive forcing warms the system, because there is more incoming or trapped energy, while negative forcing cools it, because more energy leaves the system than enters it.
In this chart, the 0 line indicates neutral radiative forcing, where there would be a balance between incoming and outgoing radiation. Bars that go to the right and are red indicate positive (warming) forcing, while blue bars that go to the left show negative (cooling) forcing components. Most of the forcing components shown here are anthropogenic, or human-caused. These include emissions of CO2, methane (CH4), and nitrous oxide (N20), among other things. Compared to the positive forcing from the sun (solar irradiance), anthropogenic positive forcings are much higher, and the average forcing from humans is around 1.5 Watts per meter squared in the positive direction. The columns on the right side of the graph show values for each forcing component, the scale at which they have an effect (do they affect the whole globe or just a local area?), and the Level Of Scientific Understanding - LOSU - (do scientists understand the component and its effects very well or not well?).