You can investigate the concentration of major GHGs in the air and correlate.

Ok

But how to ensure that this local concentration of GHG if egfectively correlated to the local rainfall change ? Bywhich mechanism ?

Very interesting question

My layman's knowledge says increase in GHGs leads to increase in surface temperature leading to changes in humidity and hence precipitation. A handbook of atmospheric sciences may help.

For instance.

In Amazonia, it is known that the équatorial forest evaporates huge amounts of humidity which in turn condensate and provoques rainfall a little bit farther.

Cutting the forest most probably reduces this evaporation and thus the rainfall farther.

And it has nothing to do with climate change.

I do not mean that I am climato skeptic, not at all, I am c9nvibced that we really experience global warming and climate change. I just wonder how to prove locally what we observe.

I think that the main point is the spatial scales of the characteristics of rainfall (i.e. its intensity and/or frequency and/or extent of wet patches etc.). The signal related to GW is never well characterized at local scale in rainfall characteristics but some regional (not yet global because of limitation in data at infradaily time scale) signals seem to emerge now for example in the intensity as illustrated by a recent huge amount of paper dealing with Clausius-Clapeyron (or what is called "super CC") mechanism linking moisture amount with temperature and occurrence of extreme rainfall. But this is till open since the amount of signal is usually lower vs the interannual or decadal variations. You may perhaps take a look for example at a recent paper discussing these issues (Article A synthesis of hourly and daily precipitation extremes in di...

I think it depends and one needs to conduct some correlation exercise to determine this. Again, rainfall being a climatic variable makes it more or less like linked together bearing processes or factors contributing to rainfall.

Thanks Vincent for the link.

It is important to bring more and more evidence of increasing occurence of extreme rainfall events, or high temperatures spells, to convince local people that climate change is ongoing. This is not easy to do. If one has some local examples to share 9t is welcome.

For instance here in Montpellier there is a request from a local association to give a conference on the impact of GW on local weather. We must think of how to explain statitics of extremes to people...

Ofcourse deforestation is one of the reasons of climate change. But I feel climate change is more of a cumulative impact of a series of local events.

I am not sure to agree really to the last comment. Climate change is primarily driven by global warming, which is massively osberved all over the planet, with local variability, as for instance the higher temperature increase at the poles than at the equator. Then this global warming provoques changes that locally people can asess in regard of their past experience.

Local people doesn't mean they are indigenous who live in harmony with their local environment. Local events of nature destruction do add up to global warming and hence climate change. You can stick to your own view but I will stick to mine.

The discussion is not one against another... You are right that local causes can bring its part to local effects of climate change.

What I would add is just that local causes would, whatever, bring local changes, and this even without climate change

And this brings back to my first question : how can we be sure that what the changes that one can observe locally is really due to climate change or is mainly/only due to local changes?

I repeat "summation of local events".

Hi Gil Mahe and others, very interesting question indeed. If you take global rainfall datasets (CRU, GPCC are admittedly only for land) and compare the anomalies with temperature anomalies, you see that global long-term changes in rainfall are nowhere close to the changes in temperature. If you take for example the 1981-2010 climatological period to calculate standard deviations, and then project anomalies in SD, you'll see that the rainfall trend in GPCC is flat, in CRU is slightly increasing, and that the temperature trend is increasing strongly (see attached graph).

What global warming thus mostly seems to do is changing where the rain falls, not so much how much rain falls in total (if you incorporate rainfall over the oceans, that may be a little different). This also underscores the importance of relative SST anomalies (relative to global mean SST anomalies).

Now the problem with climate change is that changes so far are limited in many parts of the world. The concern is not about what is, but about what is to come.

That does not mean that there are not areas where there has been very large change, but attribution is indeed somewhat complicated. I have been in an area in East Sudan that was completely barren, to be told that when people were young it was covered with trees, and had giraffes and ostriches. I've also seen areas in Darfur where a friend of mine used to get lost on the way to the well near his village when he was young, because there was so much grass. When I visited the place, the grass was not more than 10 cm tall (even in a reasonably good season). But is that variability (plus lack of recovery of rainfall post-1990), or is that post-1980s land use change (many areas were cleared of trees because people needed to sell charcoal to make a living during the drought) that prevented the rain from coming back?

The only thing that does seem to be really changing quite strongly with warming is rainfall intensity at the sub-daily scale. So far, 24-hour rainfall over most of continental Africa (away from the influence of cyclones) seems to be the same everywhere: about 80 mm for a 25 year return period, and about 100-105 mm for a 50 year return period. This also does not seem to change much yet. I've done multiple analyses of peak rainfall, and the Gumbel graphs with daily data don't change much over time. The clouds seem to be a limiting factor in how much rain they can dump. With increasing temperatures however, this rain falls in shorter periods.

Increasing rainfall intensities also mean relatively more runoff, relatively less infiltration, and thus (combined with higher temperatures and more evapotranspiration) more drought stress for vegetation with the same rainfall totals. That might at some point influence vegetation cover, crop losses, and moisture recycling patterns at the beginning and end of rainy seasons.

Very interesting.

The change in rainfall is not so obvious from GPCC data, a bit less accentuated from CRU data (but before the 40's there are onlyfew data in Africa and the methodology used by the CRU tends to give too much importance to a few stations).

Anyway, if we follow the current reasonning, higher temperatures should lead to higher humidity in the atmosphere, and thus to higher precipitations as an average over the globe.

Maybe the slight increase that we could see at the end of the time series, might be more visible in regions where the rainfall cycle may have been really enhanced recently, in regard of other regions under the influence of tropical high pressure centers for instance, which should be reinforced following the CC.

The dramatic increase in precipitation (that leads to flooding of which we have lots of examples) has to be coming from evaporation of sea water. The latter has be warmed by the increase in temperature. The ocean is WARMER, because the air is warmer. Look at this N. American winter and is slowly going away. That was not a typical winter, it was warm, compared to previous severe winters.