There are grounds for optimism; there are grounds for pessimism.
Presentationally, COP21 has been a triumph. But we will only know its the real value in retrospect. In 1815 people could not know what the Congress of Vienna would really mean for Europe, similarly the Treaty of Waitangi in 1840 and in 1919 the Treaty of Versailles. Will people look back on COP21 as the turning point - the moment real progress was made on reducing carbon emissions - or as a lost opportunity?
The developing countries sought to occupy the moral high ground. But carbon emissions have been soaring in the developing world fueled by population and economic growth - and that will continue.
There are many measurement issues. For example, China is the single largest emitter of any nation but on a per capita basis its emissions are much lower than many developed countries. Caution is also needed on the integrity of data. This year there was a massive revision upwards of China's use of coal. Can we trust the data?
COP21 is progress. Global collaboration has moved ahead with the international agreement reached in Paris signed but we should hold the champagne. We do not yet have an international treaty with binding, time bound and verifiable goals for reducing CO2 emissions and sanctions that apply if its terms are violated.