To quote David W. Wolfe, Professor of Plant & Soil Ecology at Cornell University,
“We are in the unfortunate situation of being the first generation of gardeners, ever, who cannot rely on historical weather records to tell us what our climate is, or what to expect in the future.”
Join Master Gardeners in the Arlington/Alexandria unit of Virginia Cooperative Extension in a series of monthly articles in 2021 as we explore the topic of climate change and practical actions individuals can take in their home landscapes in response. This month, we’ll present an overview.
By Elaine Mills, Extension Master Gardener
The climate change we are experiencing now is the result of global warming that occurs when the heat-trapping effect of naturally occurring gases in the atmosphere is intensified by various human activities. The most abundant of these so-called “greenhouse gases” is carbon dioxide (CO₂) that results from the combustion of fossil fuels (coal, oil, gas) and deforestation. Although less abundant, nitrous oxide (N₂O) produced during fertilizer application, fossil fuel and biomass combustion, and industrial processes is 300 times more potent and can remain in the atmosphere for 100 years. Also of concern is methane (CH₄) that is released from fossil fuel production, agriculture, and landfills. A fourth unstable, toxic gas is ground level ozone (O₃) which is produced by the interaction of nitrous oxides with volatile organic compounds (VOCs) from such products as paints, solvents, aerosol sprays, building materials, and office equipment.
There are numerous indicators of our warming world. While the temperature of air over land and water has increased, the greatest percentage of the excess heat (93.4%) has been absorbed by Earth’s oceans, resulting in more evaporation and a rise in humidity. The expansive effect of warmer water combined with the melting of glaciers, sea ice, and snow cover leads to a rise in sea level. As permafrost retreats toward the poles, tree lines shift, and plant and animal species migrate poleward as well. Increases in ocean acidity, heavy precipitation events, drought conditions, and wildfires are other conditions affected by climate change.
Our weather, measured in terms of brightness or cloudiness, humidity, precipitation, temperature, and wind, can vary on a given day. The prevalent long-term (30+ years) weather conditions, however, show pronounced upward temperature trends in every season. Just in the first months of 2020, winter temperatures in the Washington, DC, area were 6 to 7 degrees above normal. According to the November 2020 report by the National Oceanographic and Atmospheric Administration (NOAA), the global temperature for the period January to November was the second highest on record at 1.80°F above average, almost tying the record set in 2016. Scientists predict that by 2050, the climate in our region will be more like that of South Carolina and Georgia.
The Arctic is warming more than twice as fast as the global average, influencing weather patterns in the Northern Hemisphere. A weaker jet stream can bring winter “whiplash weather” with rapid shifts from bone chilling cold to unseasonably warm temperatures. In periods when less total snow is produced, deer benefit, feeding on more exposed vegetation in natural areas and gardens all winter. Lack of snow cover also impacts soil temperatures and microbial activity, affecting nitrogen retention and N₂O release into the atmosphere. On the other hand, occasional heavy snowstorms can bring damage to plants and risks of flooding from melting snow and ice.
Gardeners are more frequently experiencing “false spring” when plants emerge too early, tricked by rising winter temperatures, only to be frozen by late-season cold snaps in March and April. Gardeners may need to provide protective mulch around bulbs and perennials. The disruption in plant growth is especially noticeable in trees and shrubs. Ornamental species that bloom on old wood will lose their flowers for the year. Fruit and nut trees that break dormancy early may not have the minimum required chill hours (time exposed to cold temperatures) to produce a good crop.
The National Phenological¹ Network (NPN) measures the arrival of spring by the leaf and bloom dates of plants such as lilacs and honeysuckles, among the first to show leaves and flower each year. The network’s most recent report indicates that leaf out occurred in Northern Virginia in mid-February 2020, three weeks earlier than a long-term average (1981-2010). Changes in the bloom times of plants can affect their usual synchrony with emerging insect pollinators and migrating birds, such as hummingbirds, who seek floral nectar.
Warming summer temperatures are perhaps the most noticeable climatic change. For example, the DC metropolitan region experienced 18 days of above 90°F in 1960, 30 days above that temperature in 2017, and 61 such days in 2019. July of 2020 reported the most 90° days of any month on record, and it was the first month with nighttime temperatures that never fell below 71° since 1871. High day and nighttime temperatures cause stresses on plants. For example, trees such as sugar maple, American Beech, and eastern white pine are losing suitable habitat in their present range. Temperature extremes especially affect vegetable crops, which experience fruit set failure, bolting, and sun-scald.
Summer is also bringing a 27% increase in heavy precipitation events to our region. Examples include the record-setting amounts of rain which caused widespread flooding in Arlington and Alexandria in July 2019 and the arrival in August 2020 of Hurricane Isaias, the earliest storm on record beginning with an “I.” Not only do heavy rain and wind cause physical damage to plants from breakage, but excess amounts of water can lead to root rot and the proliferation of fungal plant diseases. Flood waters can carry pollutants, making crops such as lettuce, spinach, and kale in vegetable gardens unsafe to eat.
Prolonged periods of summer drought are an example of new stagnant weather patterns that occur when air masses remain over a geographic region for an extended period of time. There has been an 83% increase in the number of annual stagnant days since 1973. In the fall of 2019, for example, 62% of Virginia experienced moderate drought after several months of below-average rainfall. Drought conditions, combined with earlier flooding, have had a noticeable impact upon large oak trees in the region, compromising their root systems and making them more susceptible to destruction from insect pests and diseases.
The fall season is lasting at least 6 days longer in our region compared to the early 20th century. The lengthened, frost-free growing season may appear to be advantageous for vegetable gardeners, but it also gives an advantage to some non-native invasive plant species. Insect pest species also benefit, breeding additional generations before cold weather sets in. These trends will require more vigilant weed and pest monitoring. Finally, longer growing seasons combined with higher temperatures and increased carbon dioxide levels can increase pollen production, exacerbating allergic responses.
Changing seasonal patterns have other effects on plants and animals. Gardeners may observe stress-induced fall flowering of forsythias and other spring-blooming plants in response to extreme weather conditions of heat and drought during their normal growing season. The delayed migration of birds to wintering grounds results in a mismatch with ripening fruits along their route. If early winter storms come before the onset of the usual fall leaf drop, trees still carrying their leaves may be at greater risk for breakage.
Gardening these days means dealing with drought and flood conditions, less predictable periods of heat and cold, variability in hardiness zones, extreme weather events, and increased stresses from pests and disease. While the pace of climate change projected for this century can be discouraging, scientists are outlining techniques home gardeners can put into practice to adapt or mitigate its effects. The second article in this series on climate change will post on Wednesday, February 3, 2021. Please return then to read and learn about Garden Practices and Products to Minimize Your Carbon Footprint.
- Chill Seeker: Some Fruit Trees Are Very Picky about Temperatures During Dormancy, College of Agricultural, Consumer, and Environmental Sciences, New Mexico State University
- Climate Change Impacts in the United States: The Fourth National Climate Assessment, U.S. Global Change Research Program
- Climate Change Indicators in the United States , United States Environmental Protection Agency
- Climate-Friendly Gardener, Union of Concerned Scientists
- Climate-Friendly Gardening Strategies, Lecture by Dr. Sara Via, University of Maryland Extension Climate Specialist, 10th Annual Loudoun County Master Gardeners Gardening Symposium
- Fertilizer solution has become a major climate problem, Anthropocene Magazine
- Gardening in a Warming World, Cornell University Cooperative Extension
- Gardening Sustainably in an Uncertain Climate, David W. Wolfe, Cornell University
- State of the Climate (monthly summaries), National Oceanic and Atmospheric Administration
- Status of Spring, USA-National Phenology Network
- Very Warm November, TPM/IPM Weekly Report, University of Maryland Extension, December 11, 2020
- What causes flowers to bloom in the fall?, Yale Climate Connections
- What Determines When Leaves Fall?, Montana Public Radio
- What a Warming World Means for Plants, Pests, and Natural Enemies, Lecture by Michael J. Raupp, University of Maryland Extension Entomologist, MGNV Continuing Education
- USA National Phenology Network
¹Phenology (n.) a branch of science dealing with the relations between climate and periodic biological phenomena (such as bird migration or plant flowering)