The vast amount of energy affecting and driving the Earth’s climate system comes from the Sun. The Sun is mostly made up of a hot plasma of hydrogen and helium gases and creates energy by thermonuclear fusion at its core. This energy is then radiated out from the core and into the solar system as heat and light. Earth, in fact, only intercepts a very small amount of this energy.
Earth receives its energy as incoming short-wave solar (Sun) radiation. This moves downward through the atmosphere interacting with each of its layers and their components, reducing the final energy amount that reaches the surface. Some of the energy is reflected back into space. Although there is atmospheric heating from the incoming energy, it should be noted that much of this warming comes from the energy being returned from the Earth’s surface (long-wave terrestrial radiation) and through latent heat given out when water evaporates, rises from the surface and condenses.
The global heat budget is the usable energy maintained by the Earth as a balance between energy received by the planet (input) and that which is radiated back out into space (output). This balance of energy powers the atmospheric system, ocean currents, and climates which allow life to be sustained within the biosphere.
The rate of emission by the sun can vary over time. Although in a relatively stable condition (as it should be for around 3 billion years) the Sun has periods when it emits additional energy and others when it appears less active.
Extra activity on the Sun can result in huge magnetic storms called sunspots that appear on its surface as darker patches. The frequency and intensity of sunspot activity varies around and 11 year cycle and is also linked to the Sun’s 100 year cycle of brightness and strength. Periods of high sunspot activity result in increased brightness and more energy being released by the Sun and received by the Earth. Alternately, the cycle of brightness has a dimmer sun where there are very few sunspots. Changes in the Sun’s brightness, and its emission of energy, especially over longer periods, is believed to have an influence upon the climate.
Some scientists have linked the so-called Little Ice Age with a period of very low sunspot activity known as the Maunder Minimum which ran between 1645 and 1715. The sun is estimated to have been around 0.25% dimmer than it is just now and this is more than the 0.1% dimming usually found in the 11 year sunspot cycle. It would be wrong, however, to suggest that the Maunder Minimum caused this phenomenon as the little ice age started from around 1300 and blasted to approximately 1870. The Maunder Minimum does coincide with the coldest part of the Little Ice Age and may have played an aggravating part in this however more evidence is appearing to suggest that this was not a global event. The Maunder Minimum saw a larger than expected number of cold winters in Europe but this was not experienced throughout the world and some locations even higher temperatures.
At present average sunspot activity appears to be in decline and when compared to evidence from ice core samples this appears to be at a much faster rate than has been seen in the last 9000 years. If this continues Maunder minimum conditions would be reached within 50 years. For some this would suggest a return to much colder conditions and global temperatures should already be showing reduction. This is not happening and raises three possibilities:
‘ other climatic drivers are influencing the process and countering the effect of sunspots
‘ sunspots do not have as great an effect upon global climate change as previously believed
‘ sunspots have a more complex relationship with climate change which may also explain the movement regional effects observed
Recent studies have proposed that although sunspot activity can have some element of influence in global temperature change it may be a more complex relationship than originally thought. Viewing the energy from sunspot activity as inputs to, or fuelling, the climate generation system rather than merely heating up the planet could provide a better explanation for the process and an explanation for the different regional experiences. It has been suggested that not only is the quantity of energy/fuel being added to the system being changed but that different types are generated and received. Each level of the climate system adapts to this input, adjusts and even alter sits output which then has a knock-on effect on each of the following levels. Due to this it is more difficult to predict or attribute the influence of altering solar emission.
One theory, based on this more complex understanding, is that the changing level of ultraviolet light from the Sun (ultraviolet radiation levels surge during high sunspot activity) leads to an alteration in temperature distribution and winds in the stratosphere. In the Northern hemisphere this results in disruption of the jet stream and, as a consequence, colder air moves southwards (over Europe) and is held in position for much longer periods. This would explain the more regional effects previously observed.
Since the 1970s scientists have been able to monitor solar output much more accurately and without the interference of the atmosphere. This is due to monitoring by satellites. Climatologists analysed the Sun’s output and have come to believe that this natural process is not a major driver in the recent increases noted in global temperatures. Solar output has remained relatively constant and has not shown a net increase in the last 40 years and so cannot be responsible for the global warming observed.
By using sunspot records the level of solar activity can be estimated for a longer period. These records suggest that in the 20th century solar activity increased until the early 1950s but has decreased since. This evidence would allow some to infer that solar output could have influenced global temperature rises up to the 1950s but the decrease since then should have signalled a cooling. This has not happened and temperatures continue to rise.
If solar activity was driving global warming it would be expected that a uniform temperature increase would be found in each level of the atmosphere. Radiosonde balloons and satellites have provided extensive data which suggests that although the lowest layer of the atmosphere (troposphere) is experiencing warming the stratosphere is in fact cooling. Climatologists believe that this is exactly what would happen due to an increased amount of greenhouse gases being present in the troposphere creating an enhanced greenhouse effect (see later).