Climate science has now thoroughly outlined the risk associated with increasing greenhouse gases. Significant and rapid warming of the climate system is now expected to occur over the next century and beyond.
A popular argument against the enhanced greenhouse effect, or a human influence on climate, has emerged:
“We don’t doubt that climate changes, the climate is always changing”.
The statement, on the face of it, suggests that climate varies naturally. The statement also infers that the climate system is inherently unstable and is highly sensitive to a range of different natural influences.
As an argument against a human influence on climate, this one has somewhat perplexed climate scientists.
The climate always changes, especially when pushed
If climate is unstable, how does it follow that climate is insensitive to increasing carbon dioxide?
Further, if the climate system is naturally sensitive to a range of different influences, how does that make artificially doubling a (naturally occurring) atmospheric constituent a consequence free action?
The specious reasoning can be placed into context by choosing another example.
“Southern Australia is a fire prone region, and wild fires occurred naturally, long before human occupation of the continent.
"Ergo, humans cannot have any appreciable affect on the incidence or magnitude of modern day fires.”
Clearly, this statement has little credence. Humans have greatly impacted on fire regimes across Australia, for many thousands of years
The climate system does indeed change naturally, on a range of different timescales. However, natural climate variability during human history has been small in comparison to recent observed change and very small in comparison to future, projected changes.
Importantly, natural changes don’t just happen by magic. Natural forces act to push climate in a particular direction. The climate system itself doesn’t care if the push is natural or human in origin, it just feels the push.
The role of the sun in climate variability
The source of virtually all energy in the climate system comes from the sun. Geothermal and direct human heat emissions are tiny by comparison.
A change in the amount or distribution of solar radiation reaching the Earth is one of the most pivotal sources of natural climate variability. There is a range of ways that changing solar radiation affects the climate system.
The Earth’s rotation and orbit cause the most dramatic changes in solar radiation over time. The simplest of these is the difference between night and day, which is driven by a 24-hour change in solar energy as the Earth rotates on its axis.
The different seasons are also an example of solar driven climate variation.
The large swings between summer and winter are due to annual changes in incident solar radiation, primarily due to the tilt of the Earth’s axis.
The Earth’s orbital parameters also change very slightly over long periods of time. The cycle of ice ages interspersed by warm epochs is caused by very, very slow wobbles in the Earth’s orbital parameters collectively known as the Milankovitch Cycle.
Climate shifts into and out of ice-ages. This is known in climatology as the glacial/inter-glacial cycle. This cycle is associated with the largest changes in climate in the recent geological record. Global-mean temperatures change by as much as 6 degrees Celsius in response to small variations in the Earth’s orbital configuration over time.
When people compare the glacial cycle with recent human caused climate change, they often just compare the magnitude of change. Against six degrees, one degree of observed warming since 1850 doesn’t seem like much.
What gets neglected from the discussion is the rate of change and the likely magnitude of future change.
Global changes due to the glacial cycle occur, on average, around one hundred times slower than the warming observed over the last century. A very fast rate of change has a very different affect on the climate system, and the ability of natural systems to adapt to change.
Sunspot cycles and other changes
Solar radiation also changes over decades and centuries, time scales that are comparable to the human influence on climate.
The mechanisms that have perhaps received the most popular attention are solar cycles. These refer to physical solar changes which cause increases or decreases in the amount of solar energy emitted from the sun itself.
The 11-year sunspot cycle is perhaps the most widely known form of this type of solar variability.
Understanding the sunspot cycle is still an area of active research. However it is reasonably clear that a regular cycle of magnetic activity is associated with the appearance of darker regions (known an sunspots) and brighter regions (known as faculae) on the surface of the sun.
The sunspot cycle is literally a cycle in the number of sunspots, which causes solar radiation to slightly rise and fall over an 11 year period.
The existence of sunspots was known to very early astronomers, with the earliest regular observations taken in China around 2000 years ago. Modern science has been observing and recording sunspots for around 400 years, since the invention of the modern telescope. These days, satellite measurements provide very accurate observations of the sunspot cycle and associated changes in solar radiation.
Sunspot cycles can have a slight impact on global mean temperature and might even have a subtle affect on weather patterns. However to date, scientists have not found that sunspots have a regular and profound influence on the climate system.
Direct solar radiation varies on longer timescales as well. Over decades to centuries other, less well understood changes in solar magnetic activity occur. A significant decline in sunspot activity during the 17th century is today known as the Maunder Minimum, a period of reduced solar radiation.
The Maunder Minimum appears to have contributed to cooler global temperatures and a series of crop failures in parts of the northern mid latitudes.
What about the last 100 years or more?
There are a range of methods for estimating past solar radiation changes that represents an entire field of research.
Suffice to say, reconstructions of changes in solar radiation, over the 20th century in particular, are highly important to climate scientists seeking to understand why our climate has warmed.
The best way to understand how 20th century solar changes affected the climate system is with global climate models.
Changes in solar radiation in a climate model are known as solar forcing. Climate models capture the effects of solar forcing well. The most basic proof of this is that climate models reproduce the diurnal cycle (the difference between night and day) with great accuracy.
Climate models also represent the seasonal cycle in land and ocean temperatures; as well as the seasonal cycle in patterns of rainfall, pressure, winds, ocean currents and sea-ice, with impressive fidelity.
Models can also reproduce climates from the geological past, based on palaeo evidence of solar energy changes.
Using the same physics, climate models are able to include observed changes in direct solar forcing over the 20th century. To do this, they use a number of different estimates of solar forcing from different research teams.
All of the modelling conducted over the last 20 years has shown that solar changes do have a discernible affect on the climate of the last 100 years, but that those changes are typically very small compared to those associated with increasing greenhouse gases.
Finding the fingerprints
Climate scientists like to look at so-called fingerprints of climate change when examining their models to understand drivers of climate change.
They run the models with a range of different forcing experiments and examine the patterns of change associated with one or more climate influences. Then, they match those fingerprints against the climate observations.
Some of the patterns of change associated with solar forcing are similar to greenhouse gas driven changes, such as more rapid warming of the Arctic. However when the pace of change is also factored in, solar changes have been far too small to explain the dramatic warming of the Arctic that has been observed.
Other patterns of change provide a means of distinguishing between solar warming and greenhouse warming.
Perhaps the best pattern to investigate the role of the sun on the climate system is the temperature of the upper atmosphere known as the stratosphere.
If solar energy increases, so too should the temperature of the stratosphere.
Conversely, increasing greenhouse gases should cool the stratosphere, as they change the way long-wave radiation is absorbed and re-emitted through the atmosphere.
Years of study have now confirmed that the upper atmosphere is cooling, and that this cooling is consistent with both global increases in carbon dioxide and decreases in stratospheric ozone in the southern hemisphere.
On average, solar forcing has been in relative decline in recent decades, and global temperatures have continued to warm.
And into the future?
This brings us to our ability to predict how solar changes will influence climate over the next decade or century.
Stories have appeared regularly in the media predicting that we are about to enter a period of significantly reduced solar energy. Very recently, media stories have appeared suggesting an imminent return to the Maunder Minimum.
The science of predicting future changes in the sun is still relatively new. To date, media reports have been based upon very speculative information, and have not reflected the mainstream science. Importantly, the idea that a cooler sun will appreciably counteract increasing greenhouse gases has very little foundation.
While we know very clearly what we can expect for greenhouse gases based on peer reviewed climate science, our ability to predict future changes in solar radiation is limited. This is in part because solar variations tend to be slight, and also because of an inability to predict physical changes in the sun.
In this sense, it is possible that future decreases in solar forcing may reduce expected warming due to greenhouse gases.
Solar changes would have to be unusually large and sustained to cancel the effect of greenhouse gas increases. In fact, changes would have to be much larger than any observed over the last 400 years.
Even the largest conceivable changes in solar radiation (taken from those inferred to have occurred over recent geological history) would make little difference to greenhouse gas warming in the coming century. And, importantly, a cooler sun does nothing to remove carbon dioxide from the atmosphere and ocean.
Of course, the sun doesn’t just cool. It is also very possible that future increases in solar forcing may slightly amplify the effects of greenhouse gases.
Climate does vary naturally. We can’t stop the climate system changing, but we can avoid loading the dice heavily in favour of rapid warming of the climate system.
Reducing greenhouse gases is therefore not about “controlling climate”.Rather, it’s about avoiding an uncontrolled experiment on the climate system.