Science Notes

How Humans Are Changing Colorado’s Fire Season

April 18, 2022

By Melanie Thompson

Wildfire is a natural, beneficial part of the carbon cycle in Colorado. In the midst of recent destruction and fire scares it’s important to remember this. Historically, wildfire has supported the biodiversity and the resiliency of our ecosystems. Fires burn surface debris, both exposing soil and fertilizing it. They thin out trees bringing more sunlight to the forest floor to support shrubs, grasses, and flowers, which in turn support wildlife. 

However, the beneficial effects of fires rely on a balance, and human activity is tipping that balance in Colorado. Every wildfire requires two ingredients: suitable burning conditions and an ignition source, and both of these are increasing in abundance due to human activity. In the last couple decades Colorado’s climate is trending dryer, hotter, and windier leading to more days of ideal fire conditions.  Meanwhile, increasing development into previously wild areas is exposing these fuels to more human ignition sources. 

Fires have significantly departed from historic trends

The Cooperative Institute for Research in Environmental Sciences (CIRES) at CU Boulder recently released a 2022 report showing that wildfire tendencies in the US have significantly departed from historic patterns in the last 20 years. Comparing data from 1984 (the year fire records started being kept) through 2020 researchers found that starting in the 2000s fires were statistically larger, more frequent, and more widespread than the previous two decades. This was especially true in the West and Great Plains regions, which Colorado straddles.

Selected CIRES Report Data

The graphic below shows the number of wildfires in these two periods with their spatial distributions:

Image source: Virginia Iglesias, Balch, Jennifer K., Travis, Williiam R. Science Advances. (March 16, 2022) U.S. Fires Become Larger, More Frequent, and More Widespread in the 2000s

The CIRES report also cites the major reasons for the shift in fire patterns as human influence. In the last 21 years, human ignitions have caused 84% of fires in the continental US. Aridification (gradual shift from drier to wetter climate) has also doubled the extent of fire prone ecosystems in the West, at least 2/3s of which is caused by human driven warming.

So, how do these statistics take shape in the Colorado fire season? 

First: the number of fires per year and total acres burned are increasing on average.

Second: our fire season is getting longer. The number of fire weather days Colorado experiences in a given year have more than doubled in every geographic region since 1973. 

After 2021’s devastating Marshall fire, and fire scares in the early months of 2022, many have wondered whether a year-round fire season is the new norm. On this point, the data is not conclusive. Looking at the graph below it’s clear that the bulk of acres burned still occurs in the typical fire months, May through September. Although winter fires may have become more common starting in 1996, it’s hard to confirm this trend when the data only goes back to 1984. However, the USDA Forest service and fire experts agree changes in climate are making fires more likely year round.

Third: Human ignitions are responsible for majority of wildfires.

In Colorado human sources of ignition have been rising, and acres burned by human caused fires have overtaken lighting fires for the majority of acres burned in recent years. This differs from the rest of the US where, according to NASA, lightning fires, though the minority of all wildfires, are responsible for 77% of the area burned. Meaning that if Colorado can better control human ignition sources, we have a strong chance at reducing the severity of our fire season.

How is climate change affecting fires?

In the past, the Colorado wildfire season was constrained to 5 months during the summer from May to September. Snow on the ground kept fires from spreading, while an increased snowpack and lower temperatures kept fuels wetter for more of the year. Colorado’s climate has changed observably. According to NOAA, temperatures in Colorado have risen by 2.5°F since the beginning of the 20th century. Temperatures have risen in every season, especially summer and spring, and Colorado is also experiencing more very hot days and fewer very cold nights (see NOAA graphics below).

Precipitation patterns have been changing as well. Colorado has been in near constant drought since 2001, with 3 of the driest years on record in 2002, 2012, and 2018 (correlating with some of our worst fire years). We have been receiving higher average precipitation in the fall since 1980 (with the exception of 2015-2020) and lower than average precipitation in the spring since the 2000s. Snowpack has been declining slightly in Northern Colorado, and more prominently in Southern Colorado. Over the long term, climate change may cause more precipitation over Colorado winters. However, warmer temperatures mean more precipitation falls as rain, and snow melts more quickly. In Colorado we rely on gradual snowmelt to water ecosystems year-round. With less water stored in snowpack, forests and grasslands will experience longer dry spells. Leading to ideal fire conditions.

Although we are already witnessing the effects of climate change in Colorado, these effects will continue to strengthen under both lower and higher greenhouse gas (GHG) emissions pathways. If global GHG emissions continue to rise at the present rate, Colorado could see temperatures increase by 11°F by the end of the century. Even under a lower emissions pathway, annual average temperatures will likely exceed historical records by 2050. Under these higher temperatures, Colorado’s drought conditions are also projected to intensify. These changes have been and will continue to make Colorado ecosystems more prone to fires, and fires that catch will burn more intensely and cover more area than in the past.

What can we do to keep the fire season in balance?

On a state and national level:

  • Vote!
  • Write your representatives to fun forest management and prescribed burns
  • Write your representatives to support climate solutions
    • Lower emission energy sources (renewables, nuclear, natural gas)
    • Public transit and battery powered vehicles (hybrid or electric)
    • Require private companies to disclose emissions
    • Putting a price on carbon


  • Vote!
  • Discourage development in the wild urban interface (WUI)
  • Support energy efficient building codes for your area


  • Keep up to date on fire danger levels and bans 
  • Spread information that our ecosystem is more fire-prone that in the past, and people need to take precaution with campfires, debris fires, cigarettes, and equipment
  • Take steps to reduce your own carbon footprint

Here is a list of Wildfire Bills being considered in the Colorado 2022 Legislative Session: 

HB22-1012 Wildfire Mitigation and Recovery

HB22-1148 Wildfire Camera Pilot Program

HB22-1007 Assistance Landowner Wildfire Mitigation

HB22-1011 Wildfire Mitigation Incentives for Local Governments

HB-1132 Regulation and Services for Wildfire Mitigation

SB22-007 Increase Wildfire Risk Mitigation Outreach Efforts

NOAA Colorado Climate Graphics:


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