Remember that claim "global warming is causing cherry trees to bloom earlier" ? – Never mind, light pollution is likely the cause

Oh this is funny. I pointed out the other day on Twitter that “Global Warming is the universal boogeyman of the left”. Almost anything that seems slightly out of the ordinary in the natural world now seems to have global warming or climate change immediately slapped on it as the cause. I blame the education system and the lack of teaching critical thinking. One of the most famous knee jerk reactions aided and abetted by a graph that should have had a caveat from the “correlation is not causation” department comes from the Washington Post environmental reporter Jason Samenow:

His article: D.C.’s cherry blossoms have shifted 5 days earlier: what about global warming and the future? immediately puts warming as the blame, excluding any other possibility, because, see there’s this graph he made, and the evidence is right there:

My analysis of temperatures and blooms dates reveals Washington’s average March temperature has warmed 2.3 degrees in the last 90 years and that the cherry blossom peak bloom date has shifted a little more than 5 days earlier (based on simple linear regression).

In other words, real-world data support the overall idea that the D.C.’s March climate is warming and the blossoms’ bloom dates are shifting earlier in response.

He didn’t even consider any other possibility, he assumed warming was the cause from the start, set out to prove it while excluding all other possibilities, and then bolstered his preformed conclusion with “real world data”. Problem is, that’s not science, it’s activism. Dr. Richard Feynman once wrote:

The first principle is that you must not fool yourself and you are the easiest person to fool.

That’s what happened to Jason Samenow – he fooled himself, and in the process of publishing that article, fooled others.

But he didn’t look at other factors that affect trees and tree growth, he didn’t pay attention to Liebig’s Law of the Minimum. He’s in good company, neither did Dr. Michael Mann when he assumed that trees were only responding to temperature and nothing else.

Liebig’s Law applies to plant growth and can be summed up as this:

“The availability of the most abundant nutrient in the soil is only as good as the availability of the least abundant nutrient in the soil.” Or, to put it more plainly, “A chain is only as strong as its weakest link.”

For example, the growth of an organism such as a plant may be dependent on a number of factors, such as sunlight or mineral nutrients (e.g. nitrate orphosphate). The availability of these may vary, such that at any given time one is more limiting than the others. Liebig’s Law states that growth only occurs at the rate permitted by the most limiting. – Wikipedia

In the case of plants and trees, the most important basic factors in growth are:

• Water
• Soil nutrients
• Available sunlight
• Temperature

There are others, but these suffice for the point being made. We can assume that because the Washington cherry trees are curated and budding times tracked by the National Park Service that they get plenty of water and nutrients, after all we can’t have dead and dying cherry trees on the mall. That would insult Japan, who made them a gift in 1912 before they decided to surprise bomb the crap out of us at Pearl Harbor.

So we can assume water and nutrients are relatively stable in Washington where the cherry trees are – that leaves temperature and available sunlight. Samenow points out that the average temperature in Washington D.C. has gone up, citing this paper: Predicting the Timing of Cherry Blossoms in Washington, DC and Mid-Atlantic States in Response to Climate Change

That temperatures have risen in Washington D.C. is no surprise, but I argue that it is a factor of the city growth and increased local heatsinks retaining heat at night rather than global warming. For example, look at what happened in Las Vegas, another city that has grown dramatically since the 1950’s. Minimum temperatures went up, but daytime maximum temperatures have been flat.

This study just published in Proceedings of the Royal Society B says temperature alone can’t be the cause, light pollution is a major factor too, since trees also respond to increased light, be it sunlight or light pollution from a growing and glowing city. This article in Engineering and Technology Magazine summ it up well (h/t to Peter Michael Ward on Twitter).

Research by a team from the University of Exeter and independent environmental consultants Spalding Associates has combined air pollution data from satellite images with information from the public about budding trees. The comparison revealed that in areas with the highest levels of night-time light exposure, citizen scientists reported spotting emerging leaves on trees up to 7.5 days earlier.

The team focused on larger trees such as oak, ash, beech and sycamore. However, they stressed the effects would most likely be seen across the fauna and flora of affected areas.

“Our finding that the timing of bud burst of woodland tree species may be affected by light pollution suggests that smaller plants growing below the height of street lights are even more likely to be affected,” said Professor Richard Ffrench-Constant of the University of Exeter’s department of Biosciences.

…the advance in budburst across all sites is too large to be explained by increases in temperature alone.

Up to 7.5 days earlier? Gosh, that’s even worse than Samenow’s story claiming 5 days. Better yet, this data was collected by citizen scientists, according to the press release. I’ll bet some of them might even be climate skeptics.

Behavioural ecologist Peter McGregor, of the Centre for Applied Zoology at Cornwall College Newquay, said: “This study also shows that we can use citizen science in a meaningful way and that it has a real role to play in research that can have a meaningful impact.”

Have a look at the USA’s light pollution, I’ve marked Las Vegas and Washington DC for comparison:

Washington D.C. is even larger than Las Vegas, so it would seem not only would have it have similar increases in night-time temperature, but night-time light as well. This image from a 2001 paper is instructive:

Gosh, global night brightening  seems to be growing faster than global warming. And in Washington, those cherry trees sure seem to get a lot of light at night as this photo demonstrates:

Here’s the paper about trees budding earlier due to light pollution:

Note this statement from the abstract:

As light pollution is a growing global phenomenon, the findings of this study are likely to be applicable to a wide range of species interactions across the world.

Light pollution is associated with earlier tree budburst across the United Kingdom

Richard H. ffrench-Constant, Robin Somers-Yeates, Jonathan Bennie, Theodoros Economou, David Hodgson, Adrian Spalding, Peter K. McGregor

Published 29 June 2016.DOI: 10.1098/rspb.2016.0813

Source: http://rspb.royalsocietypublishing.org/content/283/1833/20160813 (open access paper)

Abstract

The ecological impact of night-time lighting is of concern because of its well-demonstrated effects on animal behaviour. However, the potential of light pollution to change plant phenology and its corresponding knock-on effects on associated herbivores are less clear. Here, we test if artificial lighting can advance the timing of budburst in trees. We took a UK-wide 13 year dataset of spatially referenced budburst data from four deciduous tree species and matched it with both satellite imagery of night-time lighting and average spring temperature. We find that budburst occurs up to 7.5 days earlier in brighter areas, with the relationship being more pronounced for later-budding species. Excluding large urban areas from the analysis showed an even more pronounced advance of budburst, confirming that the urban ‘heat-island’ effect is not the sole cause of earlier urban budburst. Similarly, the advance in budburst across all sites is too large to be explained by increases in temperature alone. This dramatic advance of budburst illustrates the need for further experimental investigation into the impact of artificial night-time lighting on plant phenology and subsequent species interactions. As light pollution is a growing global phenomenon, the findings of this study are likely to be applicable to a wide range of species interactions across the world.

Discussion

The results highlight, for the first time, to our knowledge, and at a national scale, a relationship between the amount of artificial night-time light and the date of budburst in deciduous trees. This relationship is unlikely to be caused by the UHI effect, as it is robust to the exclusion of large urban areas where temperatures are known to be elevated. Similarly, this effect is unlikely to be related to an increase in temperature alone; the maximum magnitude of effect size predicted between the brightest and darkest sites (7.5 days) is roughly equivalent to that predicted due to 2°C. Specifically, it has already been shown that urban areas are both brighter (DMSP data have been used as a proxy measure of urban extent [29,30]) and warmer (UHI effect [32]) but this is, to our knowledge, the first study explicitly investigating the relationship between the amount of night-time light and budburst while controlling for the temperature increases within urban areas. In summary, similar predictions were obtained from a model fitted to budburst data points found outside of large urban areas suggesting that it is night-time lighting causing the advance in budburst as opposed to other factors which can vary owing to urbanization, such as temperature, humidity, water availability and chemical pollution levels [38–41]. In addition, for trees experiencing average spring temperatures of 4°C, the model predicts that budburst will be advanced by up to 7.5 days in the brightest areas compared to the darkest areas.

The exposure of plants to artificial light at night is highly heterogeneous at a fine scale. Skyglow, diffuse light scattered in the atmosphere from city lights, can illuminate areas of many square kilometres to levels exceeding moonlight, but effects of artificial light on phenology have to date only been recorded as a consequence of direct illumination in the vicinity of light sources, which can be several orders of magnitude brighter [13]. As the spatial data for this study was aggregated to 5 km resolution, and the DMSP data have no direct calibration, the DMSP value for each pixel cannot be easily related to an illuminance or irradiance that any individual tree is exposed to at night. Moreover, even in dark pixels, an individual tree adjacent to a street light may be exposed to bright light, while a tree in a large unlit urban park might be relatively dark despite being located in a bright pixel. However, the DMSP pixel brightness is probably a good indication of the density of outdoor light sources, and hence the probability of any tree within that pixel experiencing a relatively high level of direct illumination; observers recording the first budbreak in three trees in close proximity will therefore be considerably more likely to be recording trees exposed to artificial light in ‘bright’ than ‘dark’ pixels.

Our finding that phenology of woodland tree species may be affected by light pollution, suggests that smaller plants growing below the height of street lights are even more likely to be affected. Such results highlight the need to carry out experimental investigation into the impact of artificial night-time lighting on phenology and species interactions. It also suggests that looking at other aspects of phenology, such as leaf senescence, would be highly worthwhile. Importantly, further studies should also try and take into account differences in light quality such as the specific wavelengths of light generated by different lighting types.

Note: about 5 minutes after publication, some minor typos, incomplete words, and punctuation errors were fixed. About 20 minute after publication, a quote exceprt was highlighted from the last sentence of the abstract. -Anthony