Terraforming is something that is, unfortunately, possible.
The first issue to address is whether or not we are /able/ to change the world through our actions, and the answer is a definitive YES.
The second is whether or not we are doing so subconsciously.
To answer that, we should look at macro and micro issues.
On the macro level, there is evidence that our carbon emissions are causing an imbalance in the atmosphere surrounding earth - for those who don't know, we have multiple layers in our atmosphere and many layers of gases that make it up. Carbon is just one of them - There are other layers that are made up of SO2, for example, which is what Volcanoes emit when they errupt (and why the argument that volcanoes cause more global climate change than humans ever could is inherently wrong). The layer of SO2 is in no danger of being overcrowded, and it has much less of an impact on solar radiation than CO2.
Which, as it happens, has a very delicate balance. Sea Plankton and Cows both emit more CO2 than humans, for example, but they also emit stable rates. Humans on the other hand tend to expand their emissions exponentially. Cows, as an example, used to have much lower populations. Now they are bred for their meat and fed food made to get them fat quickly, which has been show to increase their flatulence (#lolcowfartstudies).
We also just increase our emissions naturally as technology and industry progress and demand for products increases. We're an economic society, rather than an environmentally friendly one. The vast majority - and we're talking 99% here, which is a higher rate of certainty than the certainty of Evolution by the way - of scientists agree that man-made climate change is real and is currently affecting our planet.
On the Micro level, there is no doubt that we're having a huge negative impact. Look at Coral Reefs, the rate of species extinction attributable to humans, deforestation, coal mining, shale oil drilling, mountaintop removal, oil spills, out of control water usage, etc etc etc.
Time to put our big boy pants on and take our fingers out of our ears. The worst that could happen is that we make the earth a cleaner place.