Moving now to our second quarter earnings results, yesterday, we announced second quarter 2021 earnings of $0.80 per share.
Our earnings were down $0.18 per share from the same time period in 2020, primarily due to a change in the seasonal electric rate design at Ameren Missouri that reduced earnings $0.19 per share.
Due to the continued strong execution of our strategy, I am pleased to report that we remain on track to deliver within our 2021 earnings guidance range of $3.65 per share to $3.85 per share.
Consistent with the Missouri Smart Energy Plan, we have made significant investments to harden energy grid, which has reduced outages and installed nearly 300,000 electric smart meters for customers.
Moving now to regulatory matters, in late March, Ameren Missouri filed a request for a $299 million increase in annual electric service revenues and a $9 million increase in annual natural gas revenues with the Missouri Public Service Commission.
In our Illinois Electric business, we requested a $60 million base rate increase in our required annual electric distribution rate filing.
Our flexible and responsible plan, which includes approximately $8 billion of investments in renewable energy through 2040, the retirement of all of our coal-fired energy centers and extending the life of our carbon-free Callaway nuclear energy center focuses on getting the energy we provide to our customers as clean as we can, as fast as we can without compromising from reliability, customer affordability and the evolution of new clean energy technologies.
As you know, throughout the regulator legislative session, which ended late May, we advocated for the Downstate Clean Energy Affordability Act which was largely extended the existing framework until 2032, while putting in place provisions that would set the Ameren Illinois electric distribution ROE at the national average.
The task force will be comprised of the FERC commissioners and representatives nominated by the National Association of Regulatory Utility Commissioners from 10 state commissions.
As I discussed on the call in May, MISO completed a study outlining a potential roadmap of transmission projects through 2039, taking into consideration the rapidly evolving generation mix that includes significant additions of renewable generation based on announced utility integrated resource plans, state mandates and goals for clean imaging or carbon emission reductions, among other things.
Under MISO's Future one scenario, which is the scenario that resulted in an approximate 60% carbon-emission reduction below 2005 levels by 2039 MISO estimates approximately $30 billion of future transmission investment in the MISO footprint.
Further, MISO's Future three scenario resulted in an 80% reduction in carbon emissions below 2005 levels by 2039.
Under this scenario, MISO estimates approximately $100 billion of transmission investment in the MISO footprint will be needed.
Moving now to Page nine for an update on our $1.1 billion wind generation investment related to the acquisition of 700 megawatts of new wind generation at two sites in Missouri.
Ameren Missouri closed on the acquisition of its first wind energy center, a 400 megawatt project in Northeast Missouri in December.
In January, Ameren Missouri acquired a second wind generation project, the 300 megawatt Atchison Renewable Energy Center located in Northwest Missouri.
Turning now to Page 10 and an update on Missouri's Callaway energy center.
I am pleased to report that the generator project was executed very well and that the energy center returned to service on August 4.
The cost of the capital project was approximately $60 million.
Turning to Page 11, we remain focused on delivering a sustainable energy future for our customers, communities and our country.
Beginning with environmental stewardship, last September, Ameren announced its transformational plan to achieve net-zero carbon emissions by 2050 across all of our operations in Missouri and Illinois.
This plan includes interim carbon-emission reduction targets of 50% and 85% below 2005 levels in 2030 and 2040 respectively and is consistent with the objectives of the Paris agreement and limiting global temperature rise to 1.5 degrees Celsius.
Turning now to Page 12, looking ahead, we have a strong sustainable growth proposition, which will be driven by a robust pipeline of investment opportunities of over $40 billion over the next decade that will deliver significant value to all our stakeholders in making the energy grid stronger, smarter and cleaner.
Our outlook through 2030 does not include significant event structure investments for electrification at this time.
Moving to Page 13, to sum up our value proposition, we remain firmly convinced that the execution of our strategy in 2021 and beyond will deliver superior value to our customers, shareholders and the environment.
In February, we issued our five-year growth outlook, which included a 6% to 8% compound annual earnings growth rate from 2021 to 2025.
Importantly, our five-year earnings and rate base growth projections do not include 1,200 megawatts of incremental renewable investment opportunities outlined in Ameren Missouri's and greater resource plan.
Earnings in Ameren Missouri, our largest segment, decreased $0.18 per share, driven primarily by a change in seasonal electric rate design, resulting from the March 2020 rate order, which provided for winter rates in May and summer rates in September rather than the blended rates used in both months in 2020.
The rate design change decrease earnings $0.19 per share and is not expected to impact full year results.
Earnings were also impacted by the timing of income tax expense, which decreased earnings $0.03 per share and is not expected to impact full year results.
The increase in other operations and maintenance expenses, which decreased earnings $0.02 per share, was primarily due to more favorable market returns on the cash surrender value of company-owned life insurance in the year-ago period.
The amortization of deferred expenses related to the fall 2020 Callaway Energy Center scheduled refueling and maintenance outage and higher interest expense primarily due to higher long-term debt balances outstanding also decreased earnings $0.02 per share.
These factors were partially offset by an increase in earnings of $0.05 per share due to increased investments in infrastructure and wind generation, eligible for plant and service accounting and the Renewable Energy Standard Rate Adjustment Mechanism, or RESRAM.
Higher electric retail sales also increased earnings by approximately $0.04 per share, largely due to continued economic recovery in this year's second quarter compared to the unfavorable impacts of COVID-19 in the year-ago period.
Overall weather-normalized sales are largely consistent with our expectations outlined in our call in February as we still expect total sales to be up approximately 2% in 2021 compared to 2020.
Moving to other segments, Ameren Transmission earnings declined $0.03 per share over year, which reflected the absence of the prior year benefit from the May 2020 FERC order addressing the allowed base return on equity, which more than offset earnings on increased infrastructure investment.
Earnings for Ameren oil natural gas decreased $0.01 per share.
Ameren Illinois electric distribution earnings increased $0.02 per share, which reflected increased infrastructure investments and a higher allowed ROE under performance-based rate making.
Ameren parent and other results were also up $0.02 per share compared to the second quarter of 2020 primarily due to the timing of income tax expense, which is not expected to impact full year results.
Weather normalized kilowatt hour sales to Illinois residential customers decreased 1%.
And weather normalized kilowatt hour sales to Illinois commercial and industrial customers increased 2.5% and 2% respectively.
Turning to Page 16, now I'd like to briefly touch on key drivers impacting our 2021 earnings guidance.
And as Warner stated, we continue to expect 2021 diluted earnings to be in the range of $3.65 to $3.85 per share.
I will note that our third quarter earnings comparison will be positively impacted by approximately $0.19 per share due to the seasonal electric rate design change effective in 2021 at Ameren Missouri that we discussed earlier.
Moving now to Page 17 for an update on our regulatory matters.
Starting with Ameren Missouri, as you recall, on March 31, we filed for a $299 million electric revenue increase with the Missouri Public Service Commission.
The request includes a 9.9% return on equity, a 51.9% equity ratio and a September 30, 2021 estimated rate base of $10 billion.
In addition, on March 31, we filed for a $9 million natural gas revenue increase with the Missouri PSC.
The request includes a 9.8% return on equity, a 51.9% equity ratio and a September 30, 2021, estimated rate base of $310 million.
In late June, the ICC staff recommended a $54 million base rate increase compared to our request of a $60 million base rate increase.
Moving to Page 18.
Turning to Page 19 for a financing and liquidity update, we continue to feel very good about our liquidity and financial position.
Further, earlier this year, we physically settled the remaining shares under our forward equity sale agree for proceeds of approximately $115 million.
In order for us to maintain our credit ratings and a strong balance sheet while we found our robust infrastructure plan, we expected to issue a total of approximately $150 million of common equity in 2021 under the at-the-market or ATM program established in May.
And to date, approximately $122 million of equity has been issued through this program.
Our $750 million ATM equity program is expected to support our equity needs through 2023.
Finally, Ameren's available liquidity as of July 30 was approximately $1.8 billion.
Lastly, turning to Page 20, we are well positioned to continue to execute on our plan.

Summary:
Moving now to our second quarter earnings results, yesterday, we announced second quarter 2021 earnings of $0.80 per share.
Due to the continued strong execution of our strategy, I am pleased to report that we remain on track to deliver within our 2021 earnings guidance range of $3.65 per share to $3.85 per share.
And as Warner stated, we continue to expect 2021 diluted earnings to be in the range of $3.65 to $3.85 per share.