Scientists have sounded the alarm and even taken political positions about the levels of carbon-pollution we pump into our atmosphere and it's long term effects on global climate. In April, the National Oceanic and Atmospheric Administration (NOAA) observatory, which sits atop a 11,000-foot-tall Hawaiian volcano, released data that shows that we are now in a world where Carbon-dioxide levels are twice their historic level. (see Mashable article)
Have you ever heard of geometric feedback? You see, even though CO2 makes up a very small percentage of the total atmosphere, it is NOT the only Green House Gas (GHG). It works with methane and water vapor to keep the Earth warm at night, else we would have wild temperature swings, and freezing temperatures across the globe every day. When you add a small (25%) increase in this CO2 factor, that traps just a little more heat in the atmosphere each day, but it is accumulative.
Each day the Earth is hit by radiant energy from the Sun, equivalent to about 2.5 X 10 to the 9th Atomic Bombs (10,000 exajoules / day). Thus, keeping just a fraction of 1% of that energy / year is enough to melt the polar ice, warm the world's oceans, and evaporate enough water (including our lakes and groundwater) to add much more water vapor (a strong GHG) to the atmosphere, creating a feedback loop (I will not even mention how the melting permafrost and undersea hydrocarbons will add to the green-house effect with methane).
This "CO2 = 0.04%" is thus non-trivial. Just 2-degrees Celsius will change life on Earth as we know it, by inducing global climate change. Most of the species will not survive.