I confess that I've never paid much attention to the AGW controversy, except to note that the overwhelming majority of climatologists say there is evidence of global warming, and at least part of it could be anthropogenic.
That IS the prevailing mantra that is chanted. I'll get to this later in this post.
Do your guys say there is no global warming?
There is no gas or vapor that is capable of warming the Earth using infrared light emitted from Earth's surface. The only thing that can change the temperature of the Earth is a change in the output of the Sun.
Or if there is, that none of it is man-made,
Since we didn't make the Sun, it is not man made.
so there's nothing we can do about it?
Do about what? Assuming the same output from the Sun, there is no global warming. There can't be. I'll get to this later in the post as well.
And what makes them so sure they're right?
Ah. The key question.
To warm the Earth, additional energy is needed, that is, more energy from than the Sun is currently putting out is needed.
These 'climatologists' you hear about are also called 'climate scientists'. Trouble is, there is no theory of science about climate. These people deny science and mathematics. Their usual explanation for how the global is warming is the so-called 'greenhouse effect'.
This model touts certain gases have the magickal ability to somehow create the additional energy on Earth to warm it. Typically, the argument of the 'greenhouse effect' takes one of two forms, which I call the Magick Blanket argument and the Magick Bouncing Photon argument. Both arguments violate physics. To begin with, Earth is a bit of matter in space. All matter that is above zero deg Kelvin emits light according to the Stefan-Boltzmann law. The amount of light emitted is dependent on the temperature of the emitting surface. At the kind of temperatures found around the surface of Earth, the surface emits light just like any other matter, mostly in the infrared band (a wide band of frequencies).
The surface, being generally warmer than the atmosphere above it, loses thermal energy to that colder atmosphere. This tends to bring the surface and the atmosphere toward the same temperature. In other words, the surface is cooled by losing thermal energy to the atmosphere by simple conductive heating. Certain gases, such as CO2, water vapor, methane, etc. do absorb infrared light emitted from Earth's surface. This radiance from the surface cools the surface just like contact with cold air does. The upshot is that these gases are just another way for the surface to heat the colder atmosphere. It is heated not just by conductance, but by radiance as well.
But this does not warm the Earth. It's simply the warmer surface heating a colder atmosphere and the surface itself cooling in the process of giving up energy.
That colder atmosphere DOES become warmer by this action, but it too is made of matter, and it emits light according to the same Stefan-Boltzmann law.
All of it, the surface, the atmosphere, everything; emits light into space. Thus Earth is cooled by radiating into space, 24 hours a day in all directions, day and night.
But there is the Sun. It puts out light, including infrared light, that is absorbed by the Earth. That absorption results in heating, just like the Earth heating the atmosphere by radiance using it's own weak infrared light. This absorption is primarily by the surface, and is what makes our land and oceans nice and comfy to live on.
So energy from the Sun is equally balanced by the energy leaving Earth again.
Now enter the 'greenhouse effect'. Among the things it states, it tries to make the case that not all the energy is leaving Earth and is retained as thermal energy (what we call temperature). Unfortunately, the 1st law of thermodynamics says that you can't create energy out of nothing. Neither can you destroy energy into nothing. What comes in MUST leave. Nothing about any gas or vapor can create the additional energy needed to warm the Earth. Everything radiates light by converting thermal energy into electromagnetic energy (The Stefan-Boltzmann law). It is not possible to trap or hold light.
Another argument made is the Magick Bouncing Photon argument, which states that photons absorbed by CO2 are re-emitted back down to the surface again, heating it. Unfortunately, this effectively builds a perpetual motion machine of the 2nd order. The 2nd law of thermodynamics states that entropy cannot decrease in any system. This gives a direction for heat. Hotter areas are concentrations of energy. Colder areas are devoid of energy. That is low entropy. Heat flows from hotter areas to colder areas. In this way, energy is dispersed evenly through the system. Heat never flows from cold to hot (for that would reduce entropy). The Magick Bouncing Photon argument is literally attempting to heat a warmer surface using a colder gas in the atmosphere. According to the 2nd law of thermodynamics, this is not possible.
Another problem the 'greenhouse' effect runs into is the Stefan-Boltzmann law itself. This law states: r = C * e * t^4 where radiance is in watts per square meter of radiating surface, C is a constant of nature, e is the emissivity of that surface (how well it emits compared to a perfect emitter), and t is the temperature of the emitting surface in deg K.
By preventing light from leaving Earth, the 'greenhouse' effect is effectively reducing the radiance of Earth. At the same time, it argues, the temperature is increasing because it is 'trapped' here on Earth. The Stefan-Boltzmann law clearly states, however, that radiance is always proportional to temperature. Never inversely proportional. If temperature goes up, radiance MUST go up. If radiance goes down, temperature MUST go down with it. Both C and e are constants, C being a constant of nature, and e is a measured constant.
Thus, the 'greenhouse effect', as explained, violates the 1st and 2nd laws of thermodynamics and the Stefan-Boltzmann law.
That is how we know the whole idea of 'greenhouse effect' is bogus, and everything the 'climatologists' and 'climate scientists' say nothing more than a denial of science, for they deny each of these three laws of physics. No degree from any university, no matter it's title, makes any difference to these three laws of physics.
Thermodynamic equilibrium considers both.