A subject which you will often hear being hotly debated is whether global warming has paused, or maybe ceased altogether since around the start of the 21st Century. Some say yes, some say no. Some estimates for the length of The Pause will be longer than others, the longest estimate being that warming stopped in the mid 90's, a more typical one being from that temperatures have been stable from about 2001-present.
Looking at the data, what we do see is that the clear upward trend from 1970-1999 is no longer present in post-2000 figures. Granted it's hard to pin-down any exact date for that change of trend, and the reasons for that are twofold; the high level of randomness in climate measurements, and the occurrence of La Nina / El Nino effects round-about the end of the warming interval, causing a temperature valley and spike which hides the actual point of change. The best we can say is that the change occurred between 1998 and 2002. That is why we see so many different estimates for the length of The Pause.
Thus, although it's hard to say exactly what the trend has been so far this century, it doesn't seem to be one of a steady rise such as we saw before 2000. So, in principle yes, the clear upward trend of the late 20thC is no longer present. We have a Pause. Or, cessation of warming. We don't yet know which, and only time will give us the answer to that one.
Inevitably, various counterarguments to The Pause are brought forth. One which you will often see in unofficial blogs and the like is the drawing of a trend line across all data from 1970 to the present. On the basis of this trend line, it is claimed that temperatures are still rising. Clearly this is nonsense, and should be disregarded. Trend lines are dangerously misleading tools at best, and the golden rule is that a trend line should never be drawn across two data ranges with widely differing slopes. To do so is like measuring the gradient of a road, from a pothole to a speed hump. It tells us nothing.
Official sources tend to favour the suggestion that greenhouse-gas heat which used to remain in the atmosphere is now being accumulated elsewhere. The deep oceans being the preferred location.
The claim that the extra heat has gone into the deep oceans has a number of problems. Firstly, the commonsense observation that heat rises. It would seem unlikely for deep water to be warmer if the ocean surface shows no such effect. Remember that the greenhouse effect warms the lower layers of the atmosphere. It is therefore hard to see how the oceans could warm without the atmosphere's temperature changing. That would be a bit like turning up the heating in your top-floor flat heating only to find that it had no effect for you, but your neighbour then complained that it was making made the ground-floor flat uncomfortably hot. Counterintuitive, to say the least.
There is a scenario which could perhaps create this situation, and that is if the very cold polar oceans were becoming warmer. Most of the deep waters of the world originate as cold polar water which, because of its density, remains underneath the warmer waters from lower latitudes. Thus, a warming of the poles could create a warming of the deep oceans with no measurable surface warming. Whether this is a likely scenario is anyone's guess. Before we would accept that postulate, a mechanism would have to be postulated which transfers all of the excess heat from the whole global atmosphere into the polar seas.
The second issue is that from Vostok ice core data we know that warming of the oceans -for example at the end of an ice age- lags air temperature increases by a hundred years or more due to the huge thermal inertia of the oceans. Thus, we would not expect to see any measurable effect on the ocean over so short a timescale as a few years.
It's hard to find any accredited climate science organisations providing accurate historical data on deep ocean temperatures. Accurate data is available for 2003-on, which is when the Argo sensor network was deployed. As this analysis by Jo Nova reveals, the Argo data shows only a very small increase post-2003.
This piece from Dr Roy Spencer sheds some light on where the climate modellers arrive at their deep ocean warming conclusion from.. and it would seem to be another case of computer modelling being taken as a substitute for actual observations. Hmmm. Now, Dr Spencer is known to be something of a climate sceptic but I think I am justified in quoting his meticulous work as regards where this 'deep ocean' claim originates from. I do not in any case know of anyone who can refute that it is entirely a product of computer models. If anyone can postulate an alternative origin, I would be willing to listen.
So, to summarise, there has indeed been a pause in warming. It is claimed that the missing heat is going into the deep oceans, but on further investigation of what information that claim is based upon, it would seem to be a flimsy claim. It is also worth reflecting on the fact that a pause in warming, or indeed a slight cooling, took place between 1940 and 1970. Could this and the present trend have the same cause, I wonder?
In early 2016 we saw a powerful El Nino event push temperatures upward. Since then they have settled back. What will happen in the next few years is anybody's guess. Are temperatures are going to resume an upward trend, or settle back to the 2000-2014 level? Or, continue falling?
An important point to note is that after the 2016 El Nino, global temperatures are now cooling. We don't yet know how far they will fall, and the datasets vary quite a lot in terms of how much cooling they indicate. As of March 2018, if we take HADCRUT4 or UAH5.6 as the lower bounds, they indicate a near return to the 2000-14 Pause levels. Others indicate about half that amount of cooling from the peak. Nevertheless you will still encounter articles in the media claiming that warming continues at an ever accelerating rate! These claims are clearly wrong, and worse, may be deliberately posted to mislead the public. I suggest you calmly point out that this is incorrect. Temperatures are currently cooling.