Gardening and Climate Change
Most climate scientists agree that climate change is upon us. The year 2023 was the hottest on record, and many parts of the world saw record-breaking heat waves. Commercial agriculture will certainly see significant challenges, but what about gardeners? What can we expect as Pennsylvania gardeners, and how should we prepare?
Let’s first quickly review what we know of the causes. Greenhouse gases (GHGs) are components of the Earth's atmosphere that allow sunlight to enter but trap the heat it generates after reaching and warming the surface. The most important gases are carbon dioxide and methane, both of which naturally cycle through the growth and decay of vegetation. For thousands of years, their concentration in the atmosphere remained steady. Two centuries ago, humans tapped the power of coal and oil to build our current civilization. Because of this, we live longer and better than our ancestors, but in the past 70 years, it has become increasingly clear that using these “fossil” fuels has changed the balance of GHGs in the atmosphere. Carbon dioxide has risen from 280 ppm around the year 1800 to 420 ppm this year, and methane concentrations have more than doubled. Consequently, the Earth has warmed a bit more than 2°F since 1850—more in some parts of the world, less in others. According to the National Oceanic and Atmospheric Administration (NOAA), average temperatures in Pennsylvania have risen 2°F since 1900, and most of that rise is due to warmer winters.
Gardeners rely upon the United States Department of Agriculture (USDA) Plant Hardiness Zone Map to estimate which plants could flourish and which will likely not survive in their area. The map is straightforward. It is an estimate of the average lowest temperature in an area for the past 30 years and broken into thirteen 10°F zones: 1 being the coldest (-60° to -50°F found in some parts of Alaska) and 13 the warmest (60° to 70°F, found in some parts of Hawaii and Puerto Rico). These zones are further broken down into colder "a" and warmer "b" half-zones spanning 5°F each.
Although the USDA cautions that changes in an area's mapped zone are not necessarily the result of climate change, changes over the last three map releases (1990, 2012, and 2023) are suggestive. Between 1990 and today, the warmest areas of Pennsylvania, near Philadelphia and Delaware Counties, moved from zone 6b (-5° to 0°F) to 7b (5° to 10°F), and the coldest areas, around McKean County, moved from 4b (-25° to -20°F) to 5b (-15° to -10°F). It would not be surprising to see another half-zone shift in many places within the next 20 years.
Warming temperatures will likely benefit many gardeners with longer growing seasons and a wider range of potential cultivars, but may harm others. An example would be fruit trees requiring a certain number of winter chill hours – the hours exposed to low temperatures - to blossom and fruit. Warmer temperatures also provide opportunities for invasives or pests that otherwise would not survive colder winters.
As the Earth warms, the atmosphere holds more moisture. For Pennsylvania and, more generally, the northeastern United States, this has led to a significant increase in the number of extreme rainfall events over the past few decades. The definition of "extreme" varies from place to place, but in Pennsylvania, it's two or more inches of rain in 24 hours. Annual precipitation hasn't changed significantly, so this suggests precipitation is less evenly distributed. For gardeners, the main takeaway is to ensure gardens can absorb heavy rains without flooding or erosion. Ideally, we would also find ways to store runoff from these excess events for irrigation during dry periods.
The final piece of advice for gardeners as we adapt to a rapidly changing climate is to "plan for the unexpected." A warming atmosphere is more volatile and more likely to subject us to extremes in temperature, rainfall, drought duration, and even cold snaps. There is some evidence that the polar vortex is weakening so that, while winters are generally warmer, the occasional tongue of Arctic air can dip into the lower 48 and bring exceptional cold as it did in Texas in 2021 and again just recently. Only time will tell if events like these are rare anomalies or a growing trend.
