Since a great peak in the 90's, the global temperature has refused to rise consistently. Those in the know like erudite James Hansen, the head of NASA's Goddard Institute for Space Studies, observe, "the five-year mean global temperature has been flat for a decade." Despite the predictions about all this though, beware that before the gods destroy us, they first make us mad!
Temperatures have continued to rise in places and extremes of climate are increasing, as predicted. Other temperatures have fallen, but we are starting to understand that is just a side-effect. Greenhouse gases continue to increase in concentration in our atmosphere, but that doesn't mean science can explain the effects. The situation is complex and the rise in temperature is only one aspect of planetary change. We do not automatically understand. Science in some areas is in its infancy.
Naively, we think that the surface temperature increases by 1°C as the CO2 concentration doubles, then even more as it trebles. As the gas absorbs infra-red consistently, we can fully expect an increase from 280ppm to 560ppm to produce a single degree rise in temperature. Water vapour however and increasing cloud cover also increase global temperatures. Believe it or not, we also have other pollutants in our atmosphere. They influence the temperature change directly and also by affecting the CO2 concentration. Volcanic aerosols for example were almost ignored until their effect on historical climate was discovered.
An estimate of global warming from 2° to 4.5°C or higher, is the most established current estimate, first calculated in 2007 by the IPCC (Intergovernmental Panel on Climate Change). Drought, loss of coral biodiversity, loss of 30% of all species on earth and increased cyclone activity combined with rapidly increasing sea level all contribute to the most likely results of even a 3°C rise.
Other estimates try to compare the climate 20,000 years ago, when carbon dioxide levels were also high. Unfortunately, even they estimate rises up to 4.5°C. Talk is of stopping all the "mitigation" attempts. To think of reversing global action on warming seems very like opening the stable doors again after you got one horse to return home.
We always use simple models, despite enormous computer calculations, but that is because we don't understand all potential factors. Perhaps the climate is more sensitive to our emissions or perhaps it is less sensitive. As we measure more and more aerosol emission effects, such as soot - published in January- and check on ever-changing factors such as polar ice reduction, more evidence makes us less likely to get it wrong.
The flat period of temperature rise will be famous to future scientists. Perhaps they will find clouds don't influence warming so much, maybe having a damping effect. Perhaps the ocean currents aren't absorbing as much heat as we thought. Perhaps the deep warming of the ocean, where 30% of warming occurs in some estimates, is more significant than we thought. Perhaps natural variation of temperatures has not been taken into account correctly.
The sum of all these "perhaps" is that we only know what we know so far. As new measurement is added, we will know more, but it will never be enough. We have warmed the planet. That is all we need to act on, and act now, before none of the horses cannot be persuaded to return to the stable. Nobody remember, understands at all what would happen if temperatures rise above 4°C!