Climate Physics Chapter 1: Introduction

An overview of some of the issues considered in the study of climate physics.

What is Climate?

When we think of climate, we usually think of temperature, atmosphere, wind and rain. In fact there are a number of factors that control and regulate Earth’s climate, which has a very narrow temperature range compared to other planets. Some of these factors involve the radiation Earth absorbs and give off, the atmospheric composition, including the amount of greenhouse gases in the air, and the presence of clouds.

Faint Young Sun

Ever since early Earth cooled enough to form a solid crust 4.4 billion years ago, the Sun’s radiation has been our main source of surface heat. Although Earth’s temperature has experienced fluctuations, such as ice ages, we have experienced relative stability over time. We must not have had surface temperature fluctuations that were too large, or life would not have been able to evolve.
However, there are many factors that could cause changes in climate. Changes in the atmosphere affect the ability of radiation to enter or leave Earth. Atmospheric change can be due to bombardments that carry gases in or splash it away, volcanos that spew gases into the sky, random escape of gases due to thermal fluctuations, solar winds that blow away the outer atmosphere, chemical reactions that cause atmospheric elements to be bound in rocks, and even the presence of life. Bacteria can create methane, releasing it into the air, or pull nitrogen out of the air and fix it in the ground. Other forms of life introduce oxygen into the air via respiration, and chemical reactions with the oxygen can radically change the atmosphere.
Changes in the Sun’s luminosity also affect Earth’s climate. As the sun ages, it gets brighter, according to the formula:

\begin{equation} \mathcal{L}(t)=\frac{\mathcal{L}(t_{s})}{1+\frac{2}{5}(1-\frac{t}{t_{s}})}\\ \end{equation}

Where \(L(t)\) is the luminosity of the sun at time t, and \(t_{s}\) is the age of the sun.
This means that in the past, Earth received less solar radiation than it does now, yet it still had roughly the same climate. For this to occur, the atmosphere must have some self-regulating mechanism to counterbalance the sun, so that as the sun brightened over time the Earth’s temperature remained fairly constant.