There have been so many climate changes over the last centuries that can't always be put down to something we've been doing or not doing (see:- https://en.wikipedia.org/wiki/Little_Ice_Age). There must be something that's happened somewhere in outer space that has eventually affected our planet for better or worse.
This is something that is very much overlooked. When looked at over geological time, the Earth's climate has fluctuated wildly and some instances of change have occured fairly rapidly. There is much we still don't fully understand about solar activity and even the polarity of the Earth's magnetic field (it's 'flipped' multiple times during the Earth's existence) that it's a bit short-sighted and reductive to blame climate change solely on man made causes.
That said, there is still a very good case to argue about ending our reliance on a finite source fuel that causes major pollution to our environment and people's health. Also, stop wasting helium in balloons!
Actually we do have a good idea of what causes significant changes in Earth's climate. Ultimately it comes down to a balance between energy in and energy out. Energy in comes from the sun, energy out is heat radiated to space. Anything that changes the energy input or output will change the heat balance of the Earth and result in a globally average warming or cooling. for example, a huge volcanic eruption releases sulphur dioxide into the stratosphere, which reduces incoming solar radiation, Earth cools until the sulphur dioxide dissipates and is rained out, hence after Pinatubo the climate cooled slightly for a few years. Change the incoming solar radiation by changing the tilt of the Earth or eccentricity of the Earth's orbit, changes the seasonal climate which may, if the astronomical changes align, cause an ice age. A big El Nino results in a heat transfer from the ocean to the atmosphere, net result Earth's air temperature increases until the year after the El Nino fades, the inverse happens with La Nina. If you increase the concentration of greenhouse gases in the atmosphere, more infra-red radiation is absorbed and re-emitted to the Earth's surface instead of space, which results in an increase of radiation absorbed at the Earth's surface, global temperature rises, and as it rises, the outgoing infra-red radiation increases until a new equilibrium is reached at a higher temperature.
It is this last effect that has caused the majority of the upwards trend in atmospheric and ocean temperatures over the last 200 years. Scientists know it is not down to the sun, because solar output has remained constant or even decreased in recent decades. Changes in Earth's orbit happen over geological timescales, not a couple of centuries, so that can be ruled out. Volcanoes only cause a small cooling effect for a short time. Carbon dioxide concentrations (which is a greenhouse gas) have increased significantly since the industrial revolution, and this is by far the most likely explanation for the rise in global temperature. This CO2 rise can be attributed to human activity by looking at the isotope ratios of two different carbon isotopes in the CO2. Plants preferentially use the lower mass isotope for photosynthesis, so plant material will contain a lower concentration of the heavier isotope. The carbon released into the atmosphere by human activities is lower in the heavier carbon isotope, just like in plant material, and fossil fuels are derived from decaying plants, which gives about as good evidence as you can get that the carbon dioxide increase in the atmosphere is from burning of fossil fuels.
Yes the climate has changed in the past, sometimes rapidly, but no-one is denying otherwise. Saying the climate has always changed as a counterargument to anthropogenic global warming is like a murderer up in court saying no-one can prove he killed the victim because people have always died of natural causes. If you look at what happened during those rapid changes in climate in the past, you will find it was not pretty for life on Earth, and the best estimate is that if we don't take responsibility for our actions now, this current episode of climate change will get very ugly within the next century. Humans have been fortunate over the last 10,000 years in that the Earth's climate has been very stable, which has allowed civilisation to build up to present levels. History shows that civilisations that have large shocks applied to them tend to collapse (e.g. Vikings and Greenland), I see no reason why current humanity should be exempt from this.
There is plenty of information and rebuttal of common contrarian arguments here:
https://skepticalscience.com/argument.php
