frequently asked questions

Is climate change real?

The Intergovernmental Panel on Climate Change is an objective panel consisting of 813 experts. they assess and communicate the consensus on the state of the art knowledge related to climate change. The latest assessment report (AR5) includes 9200 peer-reviewed papers. The panel concluded that climate change is (indeed) real, they even use the word “unequivocal” (i.e. a 100% certain), which is not an easy statement to make for a single scientist (let alone 813 of them).

The reason scientists are so certain is because there is not one source of evidence. Independently from each other, observed temperature data, sea level rise, ice retreat, model simulations all show that the climate is warming.

Is it really caused by humans?

There are numerous pieces of evidence that confirm that this is true. That’s why the IPCC has used the words that this is virtually certain, meaning they are 99-100% sure. The two most compelling pieces of evidence are listed below.

1. Our climate is continually changing, though the rate of change is the important part here. Over the last 800.000 years, the climate flowing from glacial to inter-glacial periods every 100.000 years due to the changes in solar irradiation. This is because the earth’ position with respect to the sun is changes over time (via the ‘Milankovitch Cycles‘). This happens very periodically every 100.000 years. Due to this robust periodic behavior, projection into the future is simple. The next ice age (note, cooling is projected) was planned 50.000 years from now. Actually, accounting for the human influence, the has been postponed to 100.000 years from the present. Thus the amount of climate change is not unprecedented in history, but the natural fluctuations that we saw for the last 800.000 are acting on much longer time scales than we are observing now. Moreover, these changes actually project a cooling, which is unequivocally not happening.

2. Climate models can be used to study ‘counter-factual’ climate states. For example, how the climate look would look like without the increase in greenhouse gases? Off-course, models are flawed, but on a global scale, there are quite robust. Note that all 20 state of the art climate models were not at all capable of simulating the current climate change without the increase in greenhouse gases (GHG) (see figure (b)). Adding the increase in GHG to the atmosphere led to an almost perfect fit with the global mean temperature increase (see figure (a)).

Snapshot of figure from the IPCC AR5 2013 report. Panel a show the simulation with increased greenhouse gases, panel b shows the result when climate models are run while keeping the greenhouse gases constant.

A. Ganopolski, R. Winkelmann & H. J. Schellnhuber (2016). “Critical insolation–CO2 relation for diagnosing past and future glacial inception“. Nature. 529: 200–203.

Why should you study extreme events and how they change in the future?

Climate change is often expressed as the increase in the mean, for example, temperature. This is partly because we have more confidence in changes in the mean. Though the increase in the mean temperature can be devastating for e.g. ecological systems which were used to a different climate, the extreme events are ones that ‘deliver the punches’ to the system.

With this increase in the mean of e.g. 2 degrees Celsius, you would perhaps also expect an increase in the extreme temperature of 2 degrees. But this assumption is not always valid, because the climate starts behave slightly differently when it is 2 degrees warmer. Knowing how climate extremes will change in the future is vital information for numerous institutions, e.g. is your national water system resilient enough to withstand the future droughts? Extensive drought for example can cause agricultural, ecological, economic and societal damage. Understanding their mechanisms better can lead to better projections and perhaps early warnings for society.

Get in touch with us

Dr. Dim Coumou

Department of Water & Climate Risk
Institute for Environmental Studies (IVM)
VU Amsterdam
W&N-building, Room C-515
De Boelelaan 1087
1081 HV Amsterdam

Department of Earth System Analysis
Potsdam Institute for Climate Impact Research
Telegraphenberg A62, room S16
D-14473, Potsdam, Germany

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