Drought in the context of anthropogenic climate change

In the previous post I talked about European droughts association to natural modes of variability. Since this is a blog about natural versus anthropogenic climate change I thought it would be natural to follow up with a question about how anthropogenic influence on climate is affecting droughts. In particular, considering the severe drought in the US the last summers, this question is of high relevance.

A relatively newly released study in Nature by Sheffield et al. look closer at the historical record of global-scale drought trends and actually find that they have most likely been overestimated. They report high uncertainties in these trends over the past 60 years and little evidence of an increase in the total area affected by drought. One might think that it is the opposite way around in these times of a warming climate, but the climate system is highly complex and we don’t necessarily get the results that seem most logic. There exists a hypothesis in the scientific world saying that “wet is getting wetter and dry is getting drier”, meaning that the areas which normally receive a lot of precipitation will get more intense rainfall and flooding, and that areas that are already suffering from a precipitation deficit and drought will get more severe droughts. This will have huge consequences for people living in such areas.

Another study produced by a big group of scientist for the American Meteorological Society looks at six extreme events during the year of 2011 and tries to explain them from a climate perspective. Among these, they look at the severe 2011 Texas drought and ask: “Was the likelihood of either the heat wave or the drought altered by human influence on global climate?” Now considering drought over the North American continent, it is important to point out that ENSO with its cold phase La Niña, is considered to be a key driver of drought conditions in the central US (Atlas et al. 1993). In the study they use the La Niña year of 2008 as a proxy for 2011, because simulations under 2011 forcing conditions were not available, and compare to earlier decades. They find “that extreme heat events were roughly 20 times more likely in 2008 than other La Niña years in the 1960s and indications of an increase in frequency of low seasonal precipitation totals.” These findings suggest that drought is more probable now than for 40-50 years ago.

Picture from Texas drought 2011, Google

This also contribute to strengthen the “dry getting drier” hypothesis by saying that the probability of the occurrence of drought in an already dry area like Texas, is more likely with global warming.

European drought’s relationship to global SST

So far on this blog I’ve described natural modes of variability across the North Atlantic basin like the NAO, the AMO and AMOC and their related climatic impacts. They are all associated with specific climatic patterns of temperature and precipitation across large areas, and I’ve mentioned and showed illustrations of these in my previous posts. In this post I will only focus on the drought impact and try to give a summary of the factors controlling droughts across Europe.

A study by Ionita et al. (2012) looked at variability of European summer drought and its relation to global sea surface temperature (SST) by using the Palmer drought severity index averaged over the European region (see figure below).

 

This time series show the strong interannual and decadal variability of European drought. The authors show that winter SST has a strong impact in determining drought variability over Europe in the upcoming summer through different large-scale teleconnection patterns. By the use of correlation analysis they reveal the existence of three coupled modes of summer drought pattern and winter SST anomalies with different timescales:

1.    The first coupled mode represents the long-term warming trend in global SST caused by anthropogenic greenhouse gasses, in addition to a tripole-like pattern in SST resembling the positive phase of the NAO.

2.    The second coupled mode is associated with an inter-annual SST pattern in the Pacific which resembles the cold phase of ENSO (La Niña) together with the decadal fluctuation in extratropical SST resembling the Pacific DecadalOscillation (PDO).

3.    The last coupled mode is associated with strong multidecadal variability in SST across the Atlantic basin which corresponds with the AMO, also for the interannual variability. In a previous post I described how the AMO exerts a strong influence on European summertime climate, including drought. As they write in the paper: “According to Briffa et al. (2009) the summers of 1921, 1976, and 1990 were among the driest in the last 250 years, all these dry summers occurring during a cold North Atlantic phase of the AMO.”

I’ve previously described the NAO- and the AMO’s impact of heat and drought on the European climate. It now turns out that drought across Europe is associated with four different modes of variability! (NAO, ENSO, PDO and AMO). Drought is not an easy thing to define given the complexity of the phenomenon, and there also exists several types of drought. So to say that drought across Europe is determined exactly by these four modes of variability is of course just looking at the big picture.

AMO vs global warming

I ended my last blog post by asking a few questions related to the future of the North Atlantic Ocean and the AMO. What can we expect looking forward? As I also mentioned in the end of the last post, the AMO is very hard to model and therefore also to predict. Almost all models have a difficulty in simulating the AMO variability. So we really don’t know what to expect into the future…

But we do have knowledge and numerous studies which we can draw theories from. We do know that the AMO is a dominant mode of variability in the North Atlantic SST with a duration of 55-80 years (Wei & Lohmann 2012). But the question we are asking these days, and the same question I asked in my previous post, is to what extent the North Atlantic Ocean is influenced by global warming relative to the internal variability (AMO), and what consequences that may have for the future.

A study based on observational data by Wang and Dong in 2010 finds that both global warming and AMO variability make a contribution to the recent warming in the North Atlantic basin, and that their relative contribution is approximately equal. They also find (after removing a linear trend and the seasonal cycle found in observational records) that atmospheric CO₂ anomalies show a multidecadal variation approximately coinciding with the cold and warm phases of the AMO (see figure below).

They discuss that this may be related to the ocean’s CO₂ uptake through ocean circulation and the strength of the AMOC. As mentioned in my previous post, the phases of the AMO are determined by AMOC variability. There also exists a relationship between solubility of CO₂ and ocean temperature, which says that a warmer ocean leads to a release of CO₂. So a warm (cold) phase of the AMO will lead to a release (uptake) of CO₂ to (from) the atmosphere.  Summarized the warming of the North Atlantic due to AMO variability may influence global SST via the increase of atmospheric CO₂.

So based on findings from this study it seems like there exists a two-way relationship between North Atlantic Ocean temperatures and CO₂ concentrations in the atmosphere. Increased amounts of atmospheric CO₂ contributes to a warming of the North Atlantic basin. The other way around, the warm phase of the AMO with anomalously warm SSTs contributes to an increase in atmospheric CO₂, which further increases global warming. With future predictions of further CO₂ increases this does not look good. But then again the AMO is a mode of natural variability internal to the climate system which likely will change to a negative phase sometime in the future, which will reverse the picture. It is pretty clear, considering the huge climatic impacts of the AMO, that further research is needed in this field.

A shift in the European climate in the 1990 linked to the AMO

During the 1990s there was a substantial shift in the European climate. We experienced more wet summers in northern Europe and more hot and dry summers in southern Europe relative to earlier summers. What caused this anomalous shift in European climate? Was it related to anthropogenic climate change or is it just a case of natural variability in the climate system? 

A recently published paper in Nature by Sutton & Dong explains that it is the North Atlantic Ocean and a pattern of variability known as the Atlantic Multidecadal Oscillation (AMO) that is responsible for this shift in European climate. The AMO is a multidecadal variation in North Atlantic sea surface temperature (SST) which fluctuates between anomalously warm and anomalously cool phases, each lasting several decades at a time. Its instrumental record is shown below, where you can see the shifts about the mean state over the years.

As you can see there was a shift towards a positive phase in the mid 1990s, and Sutton & Dong finds evidence that this has caused the climate shift in European summers from 1996-2010. AMO-like variations in SST are closely related to the variations in the Atlantic Meridional Overturning Circulation (AMOC) (an Atlantic circulation feature part of the global ocean circulation which transports heat from lower- to higher latitudes, where the water gets so dense that it sinks and deepwater is formed, thereby overturning circulation). Evidence suggests that the 1990s warming of the Atlantic Ocean was largely caused by an acceleration of the AMOC in response to the persistent positive phase of the winter NAO, which I’ve talked about in previous posts.

As you also can see from the above figure, there was another warm period between about 1930 and 1960, with very similar pattern of North Atlantic SST as for the recent warm period. Previous research has shown that this warm state, relative to the cold period 1960-1990, forced a certain pattern of sea level pressure (SLP), surface air temperatures (SATs) and precipitation over Europe. So the similarities in North Atlantic SST anomalies between the two warm periods suggest that similar climate impacts may have been excited in the recent warm period. And that is true – very similar patterns in SLP, SATs and precipitation does exist for the two warm periods for spring (MAM), summer (JJA) and autumn (SON). This is shown in the model simulation figures below (which show anomalies relative to the intervening cool phases), which also show the typical climate pattern associated with a warm phase of the AMO:

SLP

SAT

Precipitation

This is in close agreement with the observed recent wet northern Europe summers and hot and dry southern Europe summers. It is also consistent with the observed conditions for recent springs and autumns. As Sutton & Dong explains;

“The consistency between the two warm North Atlantic periods in the patterns of anomalies in SAT, precipitation and SLP is strong circumstantial evidence that the North Atlantic Ocean was an important driver of these decadal changes in European climate.”

So what can we expect into the future? How long will the AMO stay in its warm phase, keeping the European climate locked in the same pattern? And are there also external forcings contributing to the recent warming, like anthropogenic greenhouse gasses? These are hard questions to answer given that the AMO is a natural mode of variability internal to the climate system, varying with no particular pattern, which makes it hard to predict in the future. I will try to dig deeper into this for my next blog post and further discuss these questions.

Atlantic hurricane activity and climate change

I think it is about time that I mention hurricane Sandy here on my blog. The “Superstorm” is the largest Atlantic hurricane on record and we’ve all heard about the devastating effects it had on parts of the Caribbean and the US east coast. It is easy to conclude that this unusually intense storm was caused by climate change and global warming, as a relatively newly released article in the Guardian Environment discusses.

I decided to do my own little research in the field and read a paper published in Nature by Knutson et al. (2010) on tropical cyclones and climate change. In this study they try to answer whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. First they state that there is a relationship between tropical Atlantic SSTs and the upward trend in the Atlantic hurricane activity. They then go on by looking at cyclone- (1) frequency, (2) intensity, (3) rainfall, and (4) genesis, tracks, duration and surge flooding separately. It is hard to model tropical cyclone activity because it depends on so many constantly changing factors (such as tropical SSTs), but with the improvements in models and analyzing techniques, the authors could raise their confidence level concerning cyclone-activity projections and conclude the following:

1.       It remains uncertain whether past changes in tropical cyclone frequency have exceeded the variability expected through natural causes. In the future it is likely that global mean tropical-cyclone-frequency will either decrease or remain essentially unchanged owing to greenhouse warming. Among the proposed mechanisms is the weakening of the tropical circulation.

2.       Future surface warming and changes in the mean thermodynamic state of the atmosphere (as projected by climate models) will lead to an increase in tropical cyclone intensity – both in the mean intensities and in the frequency of cyclones at higher intensity levels.

3.       Atmospheric moisture content has increased in recent decades in many regions, and will continue to increase as the atmosphere warms. This should increase rainfall rates in systems such as tropical cyclones, but that has not been established by existing studies. Tropical-cyclone-related rainfall rates are likely to increase with greenhouse warming though.

4.       There is no conclusive evidence that any observed changes in tropical cyclone genesis, tracks, duration and surge flooding exceeded the variability expected from natural causes. However, with highly confident predictions of future sea-level rise, costal environments are more vulnerable to storm-surge flooding.

      Time series of late summer tropical Atlantic sea surface temperature (blue) and the Power Dissipation Index (green), a measure of hurricane activity which depends on the frequency, duration, and intensity of hurricanes over a season. From Emanuel (2007). As you can see there exists a very close relationship.

So it seems like climate change and global warming is not directly affecting hurricane activity in the Atlantic Ocean, but more indirectly through ocean warming, sea-level rise and circulation changes. Concerning hurricane Sandy, anthropogenic activity has definitely played at least a supporting role in its intensity and destructiveness.

It was like Sandy was a sign from above to the American people, right before the election, that they and their political leaders should open their eyes and realize that climate change is happening and needs to be integrated in their politics. If hurricane Sandy had a saying in who won the election, we can literally thank God Obama won. That way there will at least be some effort to slow down the rate of anthropogenic climate change, easing future damage by superstorms like Sandy.

Tropical SST relation to the strong NAO phase

In my last post I discussed and reviewed a paper about the reason for the strong positive phase in the NAO index and the associated warming over Europe, and concluded that it is not just internal variability of NAO – forcing factors such as anthropogenic greenhouse gasses do play a central role.

I would like to point out a couple of other studies done by Hoerling et al. (2001) and Hurrell et al. (2004) that investigates another forcing factor on winter North Atlantic climate; low-latitude sea surface temperatures (SSTs). They investigate tropical SST forcing on the NAO winter index by using atmospheric general circulation models (AGCMs) forced with the observed evolution of global SSTs since 1950. They find that variance in tropical SSTs in their models simulates a positive trend in the NAO index, and the special pattern of the simulated trend agrees with that observed. They present evidence that tropic-wide changes in the atmospheric circulation associated with warming surface waters over the tropical Indian and western Pacific Oceans and associated rainfall over the tropical Indian Ocean produce a North Atlantic anomaly pattern very much like the positive index phase of the NAO.

Hurrell et al. published a part two paper (Hoerling et al. 2004) following up their first study, where they explore their previous findings. They support their earlier arguments and state that SST warming over the Indian Ocean sector is the key forcing mechanism for the observed trend in the NAO, pointing out the striking similarity in the time series of Indian Ocean and North Atlantic climate variations (see figure below).

They also discuss whether this warming contains an anthropogenic component and conclude that it most likely contains a signature of anomalous greenhouse gas forcing. They’re then using this as an argument to why models are not correctly simulating the magnitude of the NAO phase (as I described in the previous post); “Our theory for an anthropogenic, dynamical oceanic forcing of North Atlantic climate change by Indian Ocean SST anomalies might clarify why neither unforced AGCMs nor unforced coupled models are able to produce winter NAO index trends of the magnitude observed since 1950”.

So again it does look like we’re to some extent causing climate change and warming over the Euro-Atlantic part of the hemisphere with our increasing emissions of greenhouse gasses, though through a different mechanism from these papers point of view.

The NAO - natural variability vs forcing factors

Since we know that the NAO exerts a big influence on the wintertime climate over the Euro-Atlantic part of the hemisphere, one might think that winter warming in recent times is due to this natural mode of variability and that we’ve overestimated the effects of anthropogenic greenhouse gasses (GHGs) in the atmosphere. In my last post I talked about the pronounced positive trend in the NAO index and questioned whether this is part of its natural variability or if the NAO might be influenced by external forcing.

I’ve read a paper by Stephenson et al. (2006) that is trying to answer this question by looking at the response of wintertime NAO to increasing concentrations of atmospheric carbon dioxide. In the study they’ve used 18 global coupled general circulation models (GCMs) with a 1% per year increase in concentrations of CO₂. 15 of the models were able to simulate the NAO pressure dipole, which is one of its main features, but none of the models were able to reproduce a decadal trend as strong as that observed in its later time series. 14 out of those 15 models simulated an increasing trend in the NAO index with increasing CO₂ concentrations, but the magnitude of the response was generally small and highly model-dependent. Despite their different NAO responses, all the models showed a similar increasing NAO-like pattern in temperature and precipitation trends with increasing CO₂ concentrations, like warming and increasing precipitation over Northern Europe (which is fitting with my memories of “bad” Norwegian winters).

                   Norway; what I would call a "good" winter....................and a "bad" winter

Although the models in the study do suggest that the NAO show a weak positive response to increasing amounts of CO₂, the authors note that with the large amounts of model uncertainty as they found in this study “one has to be exceedingly careful about making inferences concerning future climate change”. Since the models simulate differences in NAO response, but similar responses in temperature and precipitation over Europe, the authors suggest that NAO is not the key determining factor for such changes.

So it seems like we cannot blame warming over Europe solely on the NAO's natural variability. There are so many factors interacting in the climate system and it is hard to know what is responsible for what. But it looks like there are external forcing mechanisms, including anthropogenic greenhouse gasses, contributing to the extreme winter climate over recent decades.

Various studies have investigated NAO trends and ended up with similar results. I would like to quote one of them; “[…]most authors agree that greenhouse gases are likely to be at least partly responsible for the long-term trend in the boreal winter NAO index”, Gillett et al. 2003. It is clear though, that a lot more research needs to be done on this subject to understand to what extent anthropogenic forcing from greenhouse gasses influence wintertime climate over Europe.

Recent trend in the NAO index

Below you can see the winter station-based index of the NAO, based on the difference of normalized sea level pressure (SLP) between Lisbon, Portugal and Reykjavik, Iceland. Red bars indicate positive index and blue bars negative index. (Figure found here).

As you can see from this figure, over recent decades the winter index has exhibited a pronounced trend toward a strong positive index, especially prominent around year 1990. As described in my last blog post, this positive trend corresponds to lowered surface pressure over the Arctic and increased surface pressure over the subtropical North Atlantic, with anomalously strong westerlies. I also talked about the related climatic conditions and weather patterns.

I am originally from Norway, a country thought of being a freezing cold, snow-covered place during winter. But as I look back, I have a hard time remembering those really snowy, proper winters where we could go skiing all winter long. I mostly remember grey skies, drizzling rain, and "slush" snow in the streets, which obviously is a consequence of the recent positive trend of the NAO, which typically give northern Europe mild and wet winter conditions.

So it is pretty clear that the NAO controls, in large parts, climate conditions during winter around the North-Atlantic basin. But don’t you find this recent positive index phase a bit striking? Yes, the NAO is a natural mode of climate variability, but why does it now exhibit this upward trend, prominent in its time series? To me this seems a bit “unnatural”. Are there external factors, such as anthropogenic forcing, influencing the NAO and causing this positive trend? These are questions I will be discussing in my next blog post, so stay tuned!

The North Atlantic Oscillation

I thought I would dedicate this second blog post to the North Atlantic Oscillation (NAO)– one of the leading modes of natural variability in the Northern Hemisphere (NH). It is important to get to know this phenomenon a bit better when looking into what is causing the extreme weather and climate around the North Atlantic basin, and that because the NAO has shown to exert a strong influence on the climate over large parts of the Northern Hemisphere.

The NAO is characterized by a fluctuation in sea level pressure (SLP) between the Arctic basin and the mid-latitudes. The differences in SLP drive a westerly flow that moves through the high- and low pressure systems, creating a zonal planetary-scale wave pattern across the Northern Hemisphere. Therefore the NAO is also characterized by an out-of-phase relation in the strength of this zonal flow along ~55° and 35°. The westerly flow is strongest during winter when the largest anomalies in SLP occur, as you can see in the figure below (from Hurrell et. al). Dark (light) shading indicate negative (positive) departures from the mean . So the characters of the NAO are most pronounced during the Northern Hemisphere winter months (boreal winter).

Where these high- and low-pressure systems are located, and how strong they are, determine the phase or the index of the NAO. All results are based on the winter months when the variability is largest. A high index polarity is defined as anomalous strong subpolar westerlies, with a deeper than normal low pressure over the polar region and a higher than normal subtropical high pressure . A low index polarity is defined as anomalous weak westerlies and pressure systems. A consequence of these contrasting polarities is that anomalies in climate on seasonal time scales typically occur over large geographical regions.  These anomalies in climate include surface air temperature, SST, changes in storminess and precipitation, ocean heat content, ocean currents and their related heat transport, and sea ice cover.

High index conditions are characterized by a northeastward shift and an increased intensity in the stormtrack across the North Atlantic, from northeastern North America to northern Europe, which tend to give wet and mild conditions in these areas. Stronger northerly winds over Greenland and northeastern Canada carry cold air southward and decrease land temperatures and SST over the northwest Atlantic. Warming over North America associated with the stronger clockwise flow around the subtropical Atlantic high-pressure center is also notable. Evaporation exceeds precipitation over much of Greenland and the Canadian Arctic during high NAO index winters. Drier conditions also occur over much of central and southern Europe, the Mediterranean and parts of the Middle East, whereas more precipitation than normal falls from Iceland through Scandinavia. During low index periods the weaker (and fewer) winter storms crossing the Atlantic on a more west-easterly path bring moist air into the areas surrounding the Mediterranean. Northern Europe and the eastern part of the US experiences cold air outbreaks and hence snowy weather conditions. Below you can see a (much generalized) picture of the climate patterns created by the NAO in its different phases (found on Google).

Phu, I know that was a lot in one post, but it covers the most essential information about the NAO, which is important to know in order to be able to look at how anthropogenic activity is influencing our weather versus these natural modes of variability. In my next post I will look into if/ how anthropogenic activity is influencing the NAO and what consequences that may have/are having on our weather and climate.

If you're hungry for more or want a more detailed overview of the NAO, I suggest you read the paper I already referenced to; An Owerview of the North Atlantic Oscillation.

My very first post!

Welcome to my blog! Here I will be posting news stories, scientific research and personal thoughts about the topic of the blog. I am going to discuss whether or not anthropogenic activity has got anything to do with the extreme weather events that have occurred in Europe and North America over the past years, like wet summers, flooding, heat waves, droughts, extreme winters, snow chaos and storms, or if it is solely due to the natural modes of variability in the climate system.

For example, this summer has been a season of record braking extreme weather. It has been the wettest summer in the UK for 100 years, and it has generally been an unusually wet summer across northern and central Europe. In the Black Sea region of Russia at least 103 people were killed by intense flooding caused by sudden heavy rainfall. In the US nearly two thirds of the nation experienced some level of drought, with 39 percent of the nation suffering from severe to extreme drought, destroying farmers’ crops and livestock. Colorado experienced its worst wildfire season in a decade, with half a dozen lives lost. In June Florida was hit by tropical storm Debby, which caused extensive flooding, several tornadoes, and high winds.

What is up with this weather? Can we blame it on ourselves and our emissions? How are the predictions for the future? Is it only going to get worse? These are questions I’ll be discussing further on this blog, and I hope you will find it interesting.

I will leave you with this video from the Global Climate News channel on YouTube that is trying to explain one mechanism behind the extreme climate of 2012. Although focusing on America, it will give a taste of what this blog will be about.