When people talk about climate change, often I hear the word global warming. I don't like that term because it does not fully cover and comprehensively describe what we are talking about. We're talking about changes in all components of the climate system that concern most physical variables and concern also other areas like ecosystems.
It's a nursing composing change at which we are looking now, but obviously it started from observing the temperature and that is where the origin of this term comes from.
So let's look at the temperature first. We see that over the last hundred and twenty years the Earth's mean surface temperature has increased by that point 85 degrees Celsius.
People tell me this is a very small amount. It's even less than one degree, but humans are kind of strange species, when they talk about temperature because in certain areas we have a very fine sensorium and feeling about temperature changes take for example.
If you take the measurement to the temperature measurement of your baby, it actually depends whether you measure 36.9 degrees Celsius and everything is fine, if you measure 37.0 degrees Celsius, you have identified some developing fever. So only a tenth of a degree in this temperature measurement is for us of concern, and it is similar in the Earth system by looking at the global mean temperature seeing that this temperature has risen already buy point 85 degrees and also recognizing that with the increase of greenhouse gas concentrations which today for carbon dioxide 30% higher than ever in the past eight hundred thousand years.
We are heading towards a climate that has much higher temperature in the near future if these emissions are not stopped and greenhouse gas concentrations that are not stabilized. So let us take a more comprehensive look beyond the temperatures on the long-term.
I've already talked about the composition of the atmosphere which today is very different from the natural conditions that scientists have reconstructed thanks to the analysis in polar ice cores from Antarctica we can reconstruct the greenhouse gas concentrations and note today's concentrations are very different.
This also concerns the watercycle, we are already note that in the past 60 years. The global water cycle has changed in wetter areas have become wetter and drier areas faced some more dry conditions.
We also seethat sea level has risen by nineteen centimeter itself one of the most impressive changes in the climate system concerning one of the primary resource of humans: access to land, land to build cities, land to live, but also land to produce food and that means that this resource is threatened in the future sea-level is continued.
One of the most impressive observations of climate change however is the recognition that whole world ocean has warmed up in the last fifty years.
Temperature measurements that cover essentially the entire ocean surface and also the ocean depths down to two kilometres allow us to estimate the amount of energy that has penetrated the ocean over the past 50 years. It's a staggering amount of two hundred and fifty times tend to the 21 joules an amount of energy that is almost unimaginable, but it is in front of our eyes thanks to very precise temperature measurements of the scientific community around the world.
This is an impressive manifestation that man has changed the face of the Earth, we are through the emissions of greenhouse gases caused by the burning of fossil fuels and deforestation changing the face of this planet.
When we talk about the warming I often hear people talking about the past 15 years some even talk about a warming pause. Here I think it's very important to look at the facts like the scientists, to look at the measurements and they asked questions do we understand this short-term variability of the global mean temperature that shows sometimes a large trend, sometimes a lesser trained if you look at ten years or fifteen years but, we have noted is that since 1998 if you start to calculate 15 years trend.
That trend is a bit smaller than the longer-term trend over 60 years, but it very much depends at which year you start your analysis. If you do that from 1998 you get about point zero five degrees Celsius per decade as warming trend, how'd you start to two years earlier in 1996. Your calculation would yield zero point one for the great celsius per decade, which is almost three times larger, which is a very clear indicator that looking at fifteen year trends only does not tell you much about the long-term climate change and that is why the scientist's focus on the very long time skills many decades more than a hundred years, using a a climate day today even go back for a thousand years and try to compare the climate off today with the climate over the last two thousand years with some very interesting and important insights regarding the natural variations that nature is able to create superimposed on the long-term trend that is caused by human activity.
In addition to the temperature changes that are probably the easiest to measure scientists now also can a firm large changes in the frozen world, the cryosphere, the mass of the ice sheets in Greenland and Antarctica, is shrinking at an accelerating pace. We're losing ice because of the warming in high latitudes, ice is melting and contribute substantially to sea-level rise.
We also note that the Arctic ice cover has shrunk not only in extent during summer time, but also in sickness which means a totally different quality of the ice that we are now in countering in the Arctic quality of ice that is much more vulnerable to high temperatures than much thicker ice that is able to buffer the changes.
One of the changes that almost goes unnoticed by the public is the acidification of the ocean. With the increase of carbon dioxide in the atmosphere the ocean becomes more acid, which means marine organisms that create shells that need to calcify, extract calcium carbonate, from the water they get a much harder time to build their shelves in an environment that is more acid. This acidification is one of the largest scale impacts with the longest time scale, that man is inflicting on this climate system. We know that once carbon dioxide is emitted and creates the warming in the atmosphere but also the acidification Indian Ocean. This carbon dioxide stays in the atmosphere for many millennia which means that the changes that we are bringing into the climate system today will be with us for a long long time.
Like in any scientific endeavor there are still many open questions. They don't fundamentally put into question our knowledge on climate change, but they concern important issues that we would like to know better and better understand.
I can think of three issues. The first concerns the water cycle. As water is the primary resource to humans and ecosystems, we would like to know how the water cycle responds to a warmer world. Where are the areas that we see more water. What is changing seasonality for example in precipitation. These are important questions, when it comes to trying to evaluate and estimate how ecosystems, forest, vegetation, eventually fruit production will respond to climate change and what options there are for adaptation.
The second concerns, the question of instabilities in the climate system. Observations indicate that some of the large ice streams may become unstable in the near future some already shows signs of instability in Antarctica which has serious implications for sea-level rise.
However the physical mechanisms and the observational database are still limited and I hope that in the future that the scientists together with the theoreticion understanding these processes wouldn't be able to better inform us about instabilities and proximity to which we will come in the near future.
And the third point concerns the ocean. The ocean as a vast reservoir of heat, but also carbon dioxide acidify and that effect on marine ecosystems and marine life one of the future food stores of humanity is very little known. We don't know what responsive ecosystems will be to a notion that will be more acidic and warmer that also has a change in sea level questions that need to be urgently addressed by a concerted effort of interdisciplinary action by physical scientist, biologists and chemists.
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