Isn’t climate change just a natural fluctuation?

Evidence that post-industrial global warming is largely caused by man-made CO2  is directly based on straightforward direct measurements of the atmosphere, flow of energy into the land and sea, and measurement of the physical forms (isotopes) of carbon in the biosphere and polar ice. It is further supported by evidence from advanced climate models.
 
 
The evidence for the man-made cause of 20th Century climate change is very soundly based on the following direct observations :
  1. The increase in atmospheric CO2   since 1900 correlates very closely with cumulative man-made CO2emissions and can only be explained by the strong predominance of man-made emissions over all other sources.
  2. The carbon isotope 13C is depleted in fossil fuels and plants compared with other natural sources. Decrease in the 13C/12C ratio in the atmosphere, surface water, coralline sponges with annular growth rings, Arctic and Antarctic ice, European and equatorial stalactites etc. all clearly demonstrate that the post-1750 increase in atmospheric CO2 very largely results from burning fossil fuels and forests.
  3. Satellite and surface infrared measurements show that less energy is escaping from the planet  into space at CO2 absorption wavelengths than it did 30 years ago. The brilliant Swedish chemist Svante Arrhenius predicted in 1896 that a doubling of atmospheric CO2  would lead to an eventual rise of 5°C broadly in line with today’s best predictions.
  4. Ocean and surface temperature measurements enable heat fluxes both on land and at sea to be calculated. They show that the planet continues to accumulate heat. The heat accumulation can only be explained by the strong predominance of man-made warming over other possible causes.
 

In addition a comparison of outputs from a series of different advanced climate models with actual observations indicates that surface temperature patterns, Arctic sea ice, rainfall patterns, Antarctic warming, and  changes in salinity of the subtropical North Atlantic are very likely caused by man-made emissions.

Doesn’t the medieval warm period demonstrate that 20th Century global warming has a natural cause?

There is no evidence that 20th Century climate change has the same cause as the medieval warm period. There is however strong evidence that it is largely caused by man-made emissions of greenhouse gases.

 
Even if the estimated peak temperature in the so-called medieval warm period in southern Greenland is comparable with that of the mid 20th Century temperatures inthe same locality the argument  that this proves that 20th Century global warming has a natural cause has flawed logic. If the latter type of argument was true the following argument would also be true.  There were at least as many deaths from Black Death in the middle ages as in the Second World War so the latter deaths must have been caused by Black Death. Establishing that two measurements made about 1000 years apart are similar does not establish that they have the same cause.
 
  Besides, there is insufficient evidence to claim that the pattern of climate change in time and space during the medieval warm period is similar to that of climate change since 1750.  What is known so far suggests that the observed pattern during the medieval warm period is compatible with the hypothesis that it was caused by changes in Atlantic Ocean circulation. It is widely held that changes in ocean circulation may have an impact on global climate at regional scales and changes in the equatorial Pacific circulation (El Nino) do have a small but detectable effect on global mean temperature. However there is no evidence that changes in circulation are the cause of recent global warming while the evidence outlined below strongly indicates that  man-made emissions are the primary and principal cause of this.
 

Isn’t climate change due to cosmic rays?

There is little evidence to support this idea. Trends in global warming in the recent past have been in the opposite direction to that predicted by the idea that cosmic rays cause climate change.

Cosmic rays are fast moving particles that arrive from space, partly from the Sun, and release electric charge in the Earth’s atmosphere. Laboratory experiments hint that cosmic rays could play a role in the development of tiny particles that could help clouds to form. If, and this is uncertain, the same chain of processes occurs in the atmosphere it might lead to more clouds; clouds can have a cooling effect because they reflect the Sun’s heat back into space. When the Sun is more active its magnetic field is stronger and acts to deflect more cosmic rays away from the Earth. So the argument goes that a more active Sun would lead to fewer cosmic rays reaching the atmosphere, therefore fewer clouds and hence a warmer Earth. Observations have shown that, at most, the above chain of processes produces only a small effect. Even if the effect were greater, the Sun’s activity has changed so little over the last few tens of years that it could not explain the observed rises in global surface temperature over the same period. Indeed, since the late 1980s, trends in cosmic ray intensity have been in the opposite direction to that required to explain global warming.

Isn’t climate change due to variations in the Sun’s output?

No. Although the Sun’s output does vary, its effect on global warming is about one tenth of that caused by mankind’s emission of greenhouse gases.

The primary source of energy providing heat to the Earth’s living organisms, atmosphere and oceans is the Sun. The amount of energy reaching the Earth’s surface depends on the output of the Sun, the Earth’s orbit around the Sun and effects in the atmosphere (both cloudiness and tiny particles emitted by man and volcanic eruptions). The Sun undergoes small (less than ±1 part in 2000) but significant variations in its intensity on an 11-year cycle and possibly underwent similar, longer term, changes in the distant past that cannot be measured directly. When the Sun is more active, as indicated by a greater number of sunspots on its surface, it emits more light and heat. However measurements have shown that the amount of heat reaching the Earth’s surface depends much more on the concentration of particles (aerosols) in the atmosphere than on the Sun’s activity. While there is evidence of a link between solar activity and some of the warming of the early 20th century, satellite measurements have shown that changes in the last 30 years cannot account for the recent rises in global temperatures. However, greenhouse gases can explain the rise; the warming attributable to mankind’s emissions of these gases is about ten times greater than that from changes in the Sun’s output.

The Little Ice Age and recent cooling surely mean that factors other than atmospheric CO2 are affecting the climate?

When global temperature changes over tens of years are considered, so that local and more temporary changes are averaged out, there is no doubt that global warming is happening.

It is true that the amount of carbon dioxide in the atmosphere has been rising slowly for the last 5,000 years, but this has been in response to the natural cycle driven by variations in the Earth’s orbit around the Sun. Superimposed on this are natural and local changes in climate caused for example by violent volcanic eruptions and by changes to current patterns in the oceans. However, following the industrial revolution in the 1700s, the concentration of carbon dioxide has increased very steeply. According to the latest Intergovernmental Panel on Climate Change report (2007), it is more than 90% certain that the world is already warming as a result of human activity.

The ‘Little Ice age’ of 17th -19th century Europe was not a global change, but was regional. The 20th century changes in surface temperature, by contrast, have been global. The global cooling from 1940 -1970 was caused by the injection of small particulates and aerosols into the atmosphere by polluting industries, now largely stopped by clean air legislation. Sulphur dioxide emitted during volcanic eruptions and a reduction in solar activity also contributed to this cooling.

The apparent global cooling since 1998 only appears when temperatures are averaged over less than ten years; the long-term (1998-2007) trend is still upwards. This is even clearer when a short-term warming caused by ocean currents in 1998, the El Niño effect, is removed.

The planet’s atmosphere is a chaotic system, with many complex and interconnected effects, which will cause some years to be warmer or cooler than others. So it is important to look at the trends over tens of years or longer. Some authors have examined temperature over unduly short periods of time, leading to a mistaken interpretation of the long term global trend, which is undeniably that of increasing temperature.

Hasn’t the climate varied through geological time and aren’t the present changes part of that natural variation?

The changes, and rates of change, in climate seen today are unprecedented and can only be explained by the effect of mankind’s greenhouse gas emissions.

Climate change has been occurring on Earth for millions of years. For example, the slow movement of tectonic plates and the creation and destruction of mountain chains has caused relatively large changes in temperature and sea level and affected the exchange of carbon between the Earth’s crust and the surface.

The Earth’s climate has also exhibited natural cycles of cold and warm periods which occur over thousands of years due to predictable variations in the Earth's orbit around the Sun which affect the heat reaching the Earth’s surface. In the last few million years there have also been periodic ice ages (up to 10°C cooler than present) and warmer ‘interglacial’ periods (such as the last 11,000 years). During these glacial/interglacial periods, greenhouse gas concentrations and temperature rose and fell together. Regional fluctuations of 1.0-1.5°C have also occurred; for example, northern Europe was cold until the 7th century and temperatures rose 2°C higher than today in the Medieval Warm Period (900-1300 AD).

However, today, the global concentration of carbon dioxide in the atmosphere is the highest it has been for at least the last 650,000 years. Further, the rate of increase in carbon dioxide since 1750 appears unprecedented in more than 10,000 years. Computer modelling confirms that global warming during the past 50 years was mainly caused by greenhouse gases released by mankind. It is likely that the 20th century temperature increase in the northern hemisphere exceeded that of any other century for the last 1000 years. The average global temperature also appears to be increasing faster with time. The Intergovernmental Panel on Climate Change warns of potentially large-scale, abrupt and possibly irreversible changes in physical and biological systems due to this relatively recent and rapid rise in the concentrations of carbon dioxide and other greenhouse gases. As far as we know, what is happening today has no equal in the past.

Hasn’t the climate varied through geological time and aren’t the present changes part of that natural variation?

It’s true that climate change has been occurring on Earth for millions of years. For example, the slow movement of tectonic plates and the creation and destruction of mountain chains has caused relatively large changes in temperature and sea level that have affected the exchange of carbon between the Earth’s crust and the surface (i.e. the sources and sinks of carbon). The fact that these sources and sinks are rarely in equilibrium causes the amount of the greenhouse gas carbon dioxide in the atmosphere to slowly vary and indirectly affect the global climate.

The Earth’s climate has also exhibited natural cycles of cold and warm periods which occur over thousands of years due to predictable variations in the Earth's orbit around the Sun and changes in the tilt of the Earth’s spin axis which affect the heat reaching the Earth’s surface. In the last few million years there have also been periodic ice ages (up to 10°C cooler than present) and warmer ‘interglacial’ periods (such as the last 11,000 years). During these glacial/interglacial periods, greenhouse gas concentrations and temperature rose and fell together. Regional fluctuations of 1.0-1.5°C have also occurred; for example, northern Europe was cold until the 7th century and temperatures rose 2°C higher than today in the Medieval Warm Period (900-1300 AD).

However, today, the global concentrations of carbon dioxide, methane and nitrous oxide in the atmosphere are the highest they have been for at least the last 800,000 years. Further, the rates of increase in these gases since 1900 appear unprecedented in the last 22,000 years. Computer modelling confirms that global warming during the past 50 years was mainly caused by greenhouse gases released by human activity. In the Northern Hemisphere, 1983–2012 was

likely the warmest 30-year period of the last 1400 years. The average global temperature also appears to be increasing faster with time. As far as we know, what is happening today has no equal in the past.

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