Holocene climate change and solar and other forcings | 139 ..
In this chapter we give an overview of the mechanisms causing natural climate change on decadal to millennial time-scales. Proxy climate records provide the basis for both a better understanding of the processes involved, and for testing climate models. Based on selected literature from high-resolution Holocene records of climate change, we evaluate the role of the various potential forcing factors.
Forcing mechanisms
The climate system generally can be described in terms of energy transport. The main energy source is the Sun, which emits electromagnetic radiation with wavelengths covering the full spectrum and peaking in the visible part (400 -750 nm). On its way through the atmosphere to the Earth's surface, part of this radiation is reflected, scattered, or absorbed. The absorbed energy (~70 percent) is ultimately re-emitted into space with much longer wavelengths. Since the incoming solar radiation covers only half the globe (day side) and peaks at low latitudes, there are permanent energy gradients on the Earth's surface, which the climate system tries to eliminate by transporting energy through the atmosphere and the ocean (thermohaline circulation).
All the radiative processes depend strongly on the composition of the atmosphere (gases, aerosols, dust, clouds), which provide another level of complexity, as these components are strongly connected to chemical, thermal, and dynamical changes taking place in the atmosphere on vastly differing time-scales.
Any change in these complex processes can force the climate to change. We distinguish between external (orbital and solar) and internal (volcanic and ocean circulation) forcings which we will address in the following sections.