global warming

Slow changes in the Earth’s orbit lead to small but climatically important changes in the strength of the seasons over tens of thousands of years. Climate feedbacks amplify these small changes, thereby producing ice ages.
Eccentricity

Earth’s orbit oscillates very slightly between nearly circular and more elongated every 100,000 years. This cycle is evident in the glacial/interglacial cycles of roughly the same period.

Orbital Eccentricity The Earth's orbital path varies in the degree to which it is circular. This change in its "eccentricity" varies between 0.00 and 0.06 on a 100,000 year cycle. When the eccentricity equals 0.00 the orbital path is circular and when it is 0.06 the orbital path is slightly elliptical. The current value is 0.0167.

Tilt

The Earth spins around an axis that is tilted from perpendicular to the plane in which the Earth orbits the Sun. This tilt causes the seasons. At the height of the Northern Hemisphere winter the North Pole is tilted away from the Sun, while in the summer it is tilted toward the Sun. The angle of the tilt varies between 22° and 24.5° on a cycle of 41,000 years. When the tilt angle is high, the polar regions receive less solar radiation than normal in winter and more in summer.

Earth's Tilt The Earth is tilted from perpendicular in its orientation to the Sun. This tilt varies from 22° to 24.5° on a 41,000 year cycle. The current tilt is 23.3°.

Wobble

There is a slow wobble in the Earth’s spin axis, which causes the peak of winter to occur at different points along the Earth’s elliptical orbital path. This variation in the seasons occurs on an approximately 23,000-year cycle.

Wobble of the Earth's Spin Axis The Earth's axis of rotation wobbles like a top on a 23,000 year cycle. This causes the Earth's seasons to reach their maximum at different distances from the Sun due to the elliptical shape of the Earth's orbit.

 

Small particles in the air (aerosols) may have warming or cooling effects, depending on their characteristics. Sulfate (SO₄) aerosol, for example, is light-colored and reflects sunlight back into space. The cooling effect of volcanic aerosols from the Mt. Tambora eruption of 1815 caused North America’s “year without a summer” in 1816. Sulfate aerosol is also produced by fossil fuel burning.

Black soot, which is a familiar component of urban smog and smoke from wild fires, has the opposite effect. The dark particles absorb the Sun’s energy in much the same way that dark asphalt roads become warm on sunny days.

Aerosols Can Have Different Effects Different types of small particles can have either warming or cooling effects. Sulfate aerosols released by volcanoes reflect sunlight and cool the Earth. Black soot released by smoke stacks and wild fires absorbs solar radiation and can warm the Earth. (Photo of Redoubt Volcano courtesy of USGS DDS-39)

Aerosol concentrations change for many reasons, including volcanic eruptions, spread of fires, increased windiness, drying of damp soils, changes in industrial processes, and more. Accurately projecting the extent and effect of aerosols is one of the major challenges in modeling the future 

Changes in clouds result from changes in the distribution of water vapor, temperature, and winds. The effects of global warming on these factors are complex and not well understood.

In addition, aerosols may also play a role in cloud formation. Tiny aerosol particles can “seed” clouds by providing the “nuclei” around which cloud droplets are formed. High concentrations of some aerosol types may affect the character of clouds by causing many tiny droplets to form rather than a few big ones. Clouds with more tiny droplets reflect more solar energy and tend to produce less rainfall.

Climate Is Changed by Many Processes
Climate change may result from both natural and human causes. The importance of human causes has been increasing during the past few decades.
Causes
The major causes of climate change are described in the following sections.