There are lots of naturally occurring cycles and the climate has gone through many warming and cooling phases in the past. Take a look at this plot of temperature and CO2 over the last 800,000 years. Today is on the left.
It is very easy to see that there have been lots of warming periods in the last 800,000 years. I count 13 separate times the global average goes above the baseline, including today. You may get a different number based on how you define 'separate.' We also see the level of CO2 in the atmosphere rises and falls with a very high correlation coefficient.
The denier argument goes like this: Evidence of past warming cycles proves today's warming trend is just a naturally occurring cycle. To put it succinctly, it goes like this:
There were warming periods in the past.
We have a warming period today.
The warming periods in the past were natural.
Therefore, today's warming trend is natural.
Does anyone disagree with my characterization of the denier argument?
Does anyone see the fatal flaw in this argument?
Let me give you an identical argument.
Pneumonia kills people.
Gunshot wounds kill people.
Pneumonia is a naturally occurring disease.
Therefore, gunshot wounds are a naturally occurring disease.
The problem with this argument, and the reason it is a false argument, is that it makes a false connection between the first part and the second. It is automatically assumed, without any evidence, that pneumonia and gunshot wounds are related merely because they both kill people. At no time is any evidence presented to link them together or to show that there is only one way to kill people (a naturally occurring disease).
The denier argument makes an equally false connection between past warming trends and today's warming trend. It is automatically assumed, without evidence, that today's trend and all past trends are related simply because they are both warming trends. At no time is any evidence ever presented to link them together and it is assumed, without proof, that there is only one way to cause a warming trend (a natural cycle). All of these are false arguments designed to fool and deceive. There are even multiple ways to have a naturally occurring cycle. They are not all the same simply because they occurred naturally.
This is called the fallacy of four terms.
The real truth about natural cycles is very disturbing for deniers. There are lots of naturally occurring cycles and they are studied a lot by scientists. There are the Milankovitch cycles, the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO), the North Atlantic Oscillation (NAO), the El Nino Southern Oscillation (ENSO), the solar cycle, solar activity (not the same as the solar cycle), and more.
Deniers will pull one of these cycles out with an 'Ah ha! Caught you!' type of attitude, as if they are the first person to find them and scientists are ignoring them. Ask yourself one question, who do you think discovered these cycles in the first place? Some denier playing around on his computer? No! They were discovered by scientists and we work these cycles into our calculations.
Two very influential cycles are the AMO and solar activity. Both of these give a lot of correlation to global average temperature. Unfortunately, both of these were in a negative phase throughout the warming trend of the 1980s and 1990s. The AMO has turned positive (warming), ironically during the same period deniers claim global warming has stopped (not true), but the solar activity has continued to be in a negative phase.
The sum of the natural cycles is that we are in a naturally cooling phase, not a warming one. If it was not for man made greenhouse gas emissions, the climate would be much cooler than the long-term average. All of the heating above the long-term average (actually, above what it would be without our emissions) is due to human activity in the form of greenhouse gas emissions which trap heat in the atmosphere that would otherwise escape into space.
So, for anyone claiming manmade global warming is not real because there were naturally occurring cycles in the past, you did not prove manmade global warming is not real.