"China is inappropriately made a scapegoat in this case because what causes the climate change is not today's emissions, it's today's atmospheric composition and we [USA] are primarily responsible for the excess carbon dioxide in the atmosphere - more than three times more than China and actually on a per capita basis more than an order of magnitude [i.e. ten times]. So to blame China and to say that we have to wait for them is nonsense."
Since cumulative emissions are more important than the current rate of emissions (due to the long period of time that about half the carbon dioxide we release stays in the atmosphere), then focussing simply on current rates without considering the cumulative totals obscures the bigger picture. And comparing countries with widely disparate populations also assumes that the only relevant moral unit is the nation state, a very odd assumption for individualists in the US to perpetuate. It makes much more sense to speak primarily in per capita terms. When these considerations are included, as Hansen points out, the US has little moral authority to consider China the largest contributor to the problem. Indeed, a further factor worth pondering is that roughly one-third of all China's emissions result from producing goods for western markets, so if we look at the consumption levels driving the dangerous emissions, then once again the west has little right to place the lion's share of blame for today's situation on China.
However, future emissions are also relevant, since (simplifying quite a bit), it is really the total amount of carbon dioxide humans release that matters. If we're to stay below 450 ppm (which has been the rough goal accepted by most governments - whether 450 is already too dangerous is a discussion for another day), then we've already spent more than half the carbon budget. Pre-industrial levels were around 280 ppm and we're currently at 390 ppm and rising (since more carbon dioxide is entering the atmosphere than leaving it). If this is accepted as a reasonable goal, then even if the US entirely stopped all carbon emissions tomorrow (impossible, but this is a thought experiment), China would never be able to emit as much carbon dioxide per capita as the US already has.
Does this mean that China (and other rapidly industrialising nations) bear no responsibility for their current (and rapidly rising) emissions? Of course not, but it is clear that the developed world has contributed far more to the problem than the developing world and so rightly ought to bear most responsibility for addressing it. Simply looking at current emissions obscures morally relevant considerations and enables the world's richer nations to downplay the role we have played in causing the mess. Unfortunately, such arguments are not widely understood or accepted in industrialised countries.
If you would like to see some of these statistics visualised, then Gapminder is an excellent resource. If you follow this link and click "play", you'll see a historical progression of various nations' contributions. The size of the circle indicates population size. The x-axis is per capita emissions and the y-axis is cumulative emissions. Colours are for region. You can play around with all the settings to see all kinds of relationships (a two minute tutorial is here). I haven't managed to find any graphs that compare per capita cumulative emissions. Doing so would demonstrate that China and the US are not simply in different ball parks but are playing entirely different games.