Earth has experienced high temperature long ago.
Hundreds of millions of years ago, to be exact, during the Cretaceous and Eocene periods. If you've heard this for the first time, the Cretaceous period started ~ 145.5 millions of years ago and lasted for 75 millions of years. Dinosaurs still walked the land, but modern animals and flowers started appearing. Carbon dioxide levels were close to the ones we see today. The Eocene period started 55.8 millions of years ago and lasted for 18 millions of years. Modern animals started replacing dinosaurs. At the end of the period, the weather started cooling. That's a brief summary, but i can not explain it as well as the wikipedia article on the Geologic time scale can, so if you're interested about the subject, read more there. So, there were no humans back then, the fauna we see today was just forming, so carbon dioxide could not have been the cause. So what was it?
Clouds! They've been with us all the time, covering us from sun's rays. Various factors have been tweaked while trying to simulate the pre-human super greenhouse effect: carbon dioxide, methane gas, ocean current changes, but nothing produced the required results on the models. Researchers Kump and David Pollard tried changing the so-called albedo (the amount of sunlight reflected back into space), by reducing Earth's cloud coverage. And guess what? The results were just as needed!
The researchers have an explanation for why the cloud coverage was lower back then. Apparently, changes in the the production of cloud condensation nucleus ( the particles around which water vapour condensates into rain drops) decreased the overall cloud coverage, reflecting less sunlight and allowing more to reach the earth, thus increasing the temperature. Normally, clouds reflect 30$ of sun's rays back to space. With less cloud coverage, the amount of sunlight reaching the earth is believed to have increased by 6-10%. Today, human-made aerosols and gasses serve as cloud condensation nuclei, but in the pre-human period, there were no such things. Natural gasses were the only one's that can serve as clouds' condensation nuclei, and they were correlated with the oceans. Algae are dependant on water upwelling, but the Cretaceous period is known to have had little of it, so less productivity of algae meant less natural gasses above the oceans. Less gasses - less cloud condensation nuclei, so it all eventually led to cloud insufficiency. Thus the warming started.
It may be a bit too much to swallow for non-paleoclimatologists (people who specialise in researching the weather conditions during various periods of Earth's history), but it's quite comprehensible if you give it more thought. Besides, now on a cloudy day you won't think "MOAR SUN", but "Thank you, Mr. Cloud, because of you, I won't get skin cancer".